I would love to have your players. No, no sarcasm, Flamestrike. I really am convinced this is the easiest and therefore best option.
It's just that my players would never not work to avoid this. They always have a point man, ideally an expendable one.
The end result is that the monsters almost never get that close before Initiative is rolled.
It doesn't help that D&D starts off by giving player characters better skill scores than almost all monsters. Once your Passive Perception is well past 15, non-supernatural monsters simply can't compete any longer.
Then add how D&D is chock full of goodies on top, and the deal is sealed. (Just a single example: my party has the perfect scout - it flies, it's invisible and it's expendable: a Warlock Imp familiar)
So a band of goblins would probably be smart and alert enough to shoot down the Imp, but their ambush would still be ruined.
A pack of wolves would probably not understand the Imp's noises, and would probably never become aware of the party until they're all Fireballed.
This doesn't mean I'm not on your side Flamestrike. The only difference is that I acknowledge the way the rules absolutely prevent any mystique or fog of war. Sure you can add that, but then you need to selectively ignore some rules and hope for non-optimal behavior from the group.
And by non-optimal, I mean in a deal-the-most-damage-while-taking-as-little-damage-ourselves way. In a "maximize the immersiveness and fun" way, your way definitely wins out.
For newbies and carebears the game probably works alright. It's just that to make the game fun and immersive in the hands of minmaxers and hardened murderhobos, the rules need to be made way less generous.
First, the rules establish now that as little as patchy fog or light foliage is enough to get disadvantaged perception rolls, thus -5 to passive. So the spotting distances should be pretty moderate even outside.
Now, of course, with a point man things change. It is spotting from there. That puts the point man at varying degrees of risk and you can address that with whatever encounters are appropriate. it should once in a great while be an issue.
Also, one key thing to remember is in terms of a predator fauna issue (or perhaps flora) the party may well be being tracked.
But let me boil this down to a "what is the issue here?" To me it seems your arty is doing just fine.
For that party i would definitely not be throwing "starts at 30'" without some sort of play-into-that happening. i agree with you there.
But this would be a case where, as GM, i simply think "good for you" and expect most of those kinds of encounters to be favoring them. Like you said IMP spoils ambush in the open. ("It's what i do! My middle name is Spoil in fact!" ) Seems fine. i do not, as i imagine you do not, need to concoct various kinds of narrative filler to start them at closer range in that case or to make kind of routine wilderness encounter somehow be thwarting to that... cuz its smart and should be rewarded.
Obviously challenging encounters for them that flow from the story will likely be in settings or situations which provide a lot more difficulties and constraints on a number of those things - because they are meant to be challenging and important.
And no doubt your party will be working those as well, which is fine.
And of course, the same "spot from a distance" and "invisible scouts" and so on applies to others. - against them - etc and makes sometimes them accomplishing their objectives difficult as well.
To me, as i bet it is for you, a well played party is not a problem but an opportunity to stretch my GM legs and have some fun, detailed and challenging give and take.
parties for 2x2 formation 10x10 corridors are, no offense, a dime a dozen.
"I would love to have your players." Right back at you, except that for all i know you might have been describing my gang.