Dragonblade said:
By real world what I meant was that even a Navy SEAL would get his ass handed to him if he got jumped by 20 guys in a bar.
Zhang Ziyi's character in Crouching Tiger, Hidden Dragon got jumped by 20 guys in a restaurant, and she handed them their collective asses. Clearly she is more ninja than any navy SEAL.
In the recent wuxia flick Hero, a couple of uber-twink swordsmen massacre 3,000 of the king's guardsmen in the prelude to a big showdown.
This sort of thing isn't limited to Asian fantasy either. In FOTR, Aragorn takes on dozens of orcs single-handed in the fight at Rauros, and later in TTT, he and Gimli fight another huge horde at the gates of Helm's Deep.
In silver age comics, superheroes (and villains) can often level entire city blocks when they fight, but they still work within the framework of society.
Basically, D&D tries to emulate this aspect of cinematic fantasy, where individual heroes can often take on hordes of mooks and come out on top. It may not be realistic, but it's a genre convention that nearly everybody is familiar with. Whatever you may think about hit points, few other systems are as good for facilitating epic violence. In fact, hit points are so good for this that they often show up in genres other than fantasy, especially in computer games. Fallout had hit points (although it gave the player the option of circumventing them via called shots); so did Jagged Alliance; so do many FPS's.
Of course, if you _don't_ want epic violence in your game, then hit points won't be to your taste. But that probably says more about what you want in your game, than about hit points.
As for why these 20th level characters don't trash every town they come across: they just don't. In fiction, powerful characters often still have a social and cultural framework within which they operate, even if there's noone with a big stick watching over them. Aragorn might have been able to take on everyone in Theoden's hall, but that's not what he's about. The flying swordsmen in Chinese wuxia stories can take on gangs of mooks, but they aren't interested in becoming tyrants: that's for the bad guys. In this sense, high-level D&D is best thought of as an extremely flexible medium for spinning tales of heroic fiction -- storytelling -- rather than an exercise in simulating reality.
Basically if you want to play a complete sociopath because you have uber-powers, then sure, there may be little standing in your way (except similarly high-level villains). However, nothing's forcing you to be a sociopath, and in fact, you'll have far fewer headaches if you don't try.