This is not strictly true. There are general fatigue rules, at least in 3.X, which cover the effects of strenuous labor over time.
Having just searched the SRD, this isn't true. There are the conditions
Fatigued and
Exhausted. There are also things that can trigger Fatigued (and fatigued twice is exhausted). The mundane ones are sleeping in armour, running long distance too much, and cold, heat, or starvation. (Oddly enough swimming by the RAW doesn't).
And that you can house rule to make things work isn't enough to cover the gap in the rules.
It's not so much the case that there is nothing in the system unless it is modeled by the game engine. It's more that only the rules of physics can reasonably be codified.
This isn't true. Codifying the physics is generally not terribly reasonable (see hit points for details) and other rules can reasonably be codified although it's technically more challenging (see below).
When it comes to things like morale and inspiration, though, less really is more. If you codify something like that, then you get people playing the system rather than the game. Yo u know the big alignment debate, over whether enforced alignment helps or hinders roleplaying? It's kind of like that, but to an even greater extent. If you get points for being selfless, then you wind up with players acting selfless, not because it's true to the character, but so they get the points. Any codified system can be solved, so unless you want players to "optimize their roleplaying", you must leave the roleplaying mechanics un-codified.
You're talking about alignment mechanics as if they are a good example of ... anything. And they aren't. Good personality mechanics assume two things.
1: They can be solved.
2:
Playing in line with the solution leads to a more interesting game and to "Optimised roleplaying" that's almost indistinguishable from character driven roleplaying.
Let's take some actual games here rather than some straw games. The obvious first game is
Fate Core and its Aspects system. The aspects are five descriptive statements that reflect the character - and in order to gain a bonus Fate Point you need to act in line with your aspects in such a way that it harms or impedes your character. You could only ever gain a bonus from acting selflessly if your character had
Selfless (or
Generous to a fault or whatever) as an aspect - and it lead to your character being put in a pickle. If your character has
Steals Candy from Babies as an aspect then you get absolutely nothing out of acting selflessly, but you gain a Fate Point when you get in trouble for stealing candy from babies. (Note: Not when you steal candy from babies - but when the GM puts you in a spot because of it).
So as you can see you do not get a bonus for acting selflessly in Fate unless acting selflessly is a part of your character. You are acting selflessly because the way you have set up the character encourages you to do so to fish for Fate Points. So unless you care enough about motivation that rules that encourage the greatest power gamer on the planet to roleplay the character almost the way an intense character-roleplayer would is a bad thing then I don't see the problem here.
Now for our second game we're going to take Smallville. In Smallville the most powerful move you can make is C
hallenging your values or
Challenging your relationships with people you care about but you can't do that often because it weakens you for the rest of the session - and you need to be using those values straight most of the time to roll them. So how do you optimise that? You create a




ed up and twisted character with enough angst to satisfy any drama major who over the course of the season finds that their understanding of themselves, of other people, and of reality is wrong. And then play that to the hilt, tangling with, feuding with, helping, and learning about the other PCs. If tangled up bundles of angst and misunderstanding are what you want then this is playing the game perfectly. If not you shouldn't be playing Smallville at all.
For our third we're using Monsterhearts. The key thing about Monsterhearts is that all the moves you start off with are bad and likely to blow up in your face. You're a teenager who never mind not understanding the world doesn't understand themselves. How do you gain experience? Of your four stats, one will be highlighted by the GM and one by one of your fellow players. And you can gain 1XP/scene for using one of those stats. But using the moves based on those stats is likely to send you on a highway to hell because the moves are generally counterproductive unless you completely nail the roll. Also each monster type is pointed in a certain direction by their monstrous moves - and being monsters the way they are pointed is nowhere good, from the werewolf turning into an unstoppable wolf man to the vampire toying with their victims. So how do you optimise using the Monsterhearts personality mechanics? Take the game on a highway to hell, crashing and burning in the way the other players think would be the most entertaining, and using the abilities your being a monster gives you to further enhance the melodrama as you try to drag yourself out of the mess you are busy making. Or to make it even worse. (On a tangent, Monsterhearts and the rest of the Apocalypse World based games are entirely without disassociated mechanics - making D&D 3.X look positively disassociated by comparison).
So your case only appears to hold water if you have only ever seen games with bad personality mechanics like alignment. Yes, less is more. But none at all is nothing at all (which admittedly is an improvement on alignment).
(Yes, I know I simplified all three games madly).