Easy answer: most players (and characters) when given options will, all else being equal, trend toward the path of least resistance. Here, that path is to sit back and let others take the risk.
The character who rarely if ever joins in the derring-do is, by law of averages, going to outlive the rest of her party (and probably their replacements as well) by a wide margin; all the while slowly but surely accreting wealth and xp the others don't have.
Eventually one hopes the passengers will realize why they're falling behind and join in more often.
What I've seen happen in the past goes roughly like this. This is an abbreviated version where things happen quickly, to show my point; but imagine this spinning out slowly over a 20-adventure campaign:
Party forms with 6 characters A-B-C-D-E-F, let's say character A is the passenger. They go out in the field and after their first adventure their status is:
A - alive, full share of xp and loot*
B - alive, full share of xp and loot*
C - alive, full share of xp and loot*
D - alive, full share of xp and loot*
E - dead
F - dead (this, IME, would be my guy)
* - loot includes possessions of dead companions E and F, along with what the adventure gave.
So, a couple of recruits G and H come in to replace E and F and off this meery crew goes. After their second adventure they look like:
A - alive, full share of xp and loot (so now has 2 total shares of each)
B - dead
C - alive, full share of xp and loot (so also 2 of each)
D - alive but missed half the trip, so half share of xp and loot (thus 1.5. total of each)
G - alive, full share of xp and loot (1 total of each)
H - alive, full share of xp and loot (1 total of each) (my replacement for F - made it through an adventure, yippee!)
And they're all richer by sharing out B's stuff.
So now character I comes in to replace B, and off we go on adventure 3. Results afterward:
A - alive, full share of xp and loot (so now 3 and 3 overall)
C - alive, full share of xp and loot (so now 3 and 3 overall) - but at this point C retires, having had her fill of all this.
D - alive, full share of xp and loot (so now 2.5 and 2.5 overall)
G - alive, full share of xp and loot (so now 2 and 2 overall)
H - dead (I lose another one...)
I - alive, full share of xp and loot (so now 1 and 1 overall)
And a fourth adventure includng new recruits J and K is all we need to make my point clear - off they go, back they come:
A - alive, full share of xp and loot (so now 4 and 4 overall)
D - dead
G - alive, full share of loot and xp (so now 3 and 3 overall)
I - alive, full share of xp and loot (so now 2 and 2 overall)
J - alive, full share of xp and loot (so now 1 and 1 overall)
K - alive, full share of xp and loot (so now 1 and 1 overall) (my third try at this...)
Character A, by simply being a passenger while the rest of the party took the risk, is now both the wealthiest and the highest-level character in the group; with wealth even further enhanced by sharing in the loot from her dead companions. Assuming each adventure produced a vaguely-equal treasury, A might be 4 times as wealthy as J and K after the 4th adventure. Yet A has achieved this by intentionally sitting back and letting others take the risks, sometimes to their demise.
The level difference can be mitigated by the DM allowing new characters to come in at a particular level relative to the party average or whatever...but the wealth aspect cannot, as treasury division is almost universally a player-driven element. The DM can introduce new characters with some possessions, sure, but has to be wary about those possessions simply adding to the wealth of others when the new recruit dies.
The passenger player might be enjoying it but the rest of the crew might very well end up feeling resentful, particularly if my example above starts coming out real.
Lanefan
Hmmm... some specific responses...
first bold - in my experience the players' characters tend to take the path of most fun and sitting there at the other table while the players go into the mission is not usually that. But if that one character wants to "sit at the inn" or "hold the horses" and the player is fine with that and the player's are fine with that, why should i give them less advancement - why is my game better?
second bold - i assume that if you have told them how xp is given and they are sentient they realized the result before they went that way - knew it from the start - and if its fun why would they stop? How important are levels to the guy sitting back at the inn?
As for the bit about law of averages and accruing wealth - seems to me the ones going out are accruing the wealth and XP as well. The guy sitting back does not accrue any wealth if the others dont come back out with success.
As for your own particular chain of dead guys sequence, a lot of that depends on the way death and replacement is handled and especially the frequency. i would suggest to you that it is *not8 the passenger syndrome that is causing whatever it is that you are seeing as the problem but the death rate and replacement rules and gear sequence combining to make it a non-satisfying event.
Now, obviously one could also point out that had the extra character actually gone in, the odds of success go up even more, the death rate goes down and he winds up even more well off.
See, the trade-off for the passenger is that he stays home and safe but he gains less loot overall as the team fails more often. this can be even more relevant once they get to the point where dead equals expensive raising and cutting into profits.
But, as to how resentful this and that... honestly there are a lot of things you can gain Xp for that can be equally "resent producing" behavior. i would think that once the group of more outgoing get tired of this, they will change the deal rather than just keep going along with it.
again, not the fault of the Xp award per se.
Foe me, for longer than i care to remember, i have not linked advancement to things like specific Xp goals or tasks so whenever i hear how Xp is needed to avoid folks doing abc... i think back to "is this what they do in games where XP is not given that way" and i typically find the answer to be "nope."
The "nero wolfe" you describe - there are lots of games without xp tied to actions etc or even xp based advancement at all and yet we just dont see that as some massively common trope.
primarily because, **IMO and IMX**, players show up to play not watch others play while they wait to get paid so they can do more inventory.
But, if advancement was not linked to Xp, if replacement characters came in with comparable gear and loot, etc then there would not be this "richest guy is Nero wolfe" thing going on.
Would your game be better if "nero wolfe" were levels behind the others because you choose to cut back or cut out their XP?"
if not and its really just you wanting to punish them for not "getting on board" with the playstyle you want to impose... why not just be upfront and say "nah, not going to allow that" at session zero?
IMX, having players play their characters as part of the collaborative effort works fine without an XP cudgel to shackle their options.