I'll probably be lynched for this...
RPGs, like books, comics, TV, movies, video games and other types of "heroic" media is a form of escapism. We (being people who engage in such endeavors) use the to escape the modern hum-drum world we live in day-in-day-out. Most of such media allows for different types of escapism (romance novels provide one type, Mario Party provides another, Porn provides a third type, you get the idea). D&D might be about a medieval warrior cleaving orcs with his sword and V:TR might be about angst-ridden monsters of the night, but at the end of the day, its about being something different than Bob the Accountant or Betty the Teacher.
D&D (in specific) focuses on the concept of power-acquisition. Typically, D&D has involved overcoming challenges to gain increased power. The challenges typically are combat-based; its roots are still in war-games. The power increase comes from gold, magic, and experience points. Its a game of tangible goals (stopping the orc-horde, reaching the next level of dungeons) and tangible rewards (+X swords, new levels with increased statistics).
Not all RPGs are built around such endeavors; Vampire seems much more interested in the introspection of being the monster; while a game like Dogs in the Vineyard actively seek to make combat a non-starter.
Certainly, the combat aspect of D&D can be reduced (though as long as a "fighter" class remains, I doubt it could be eliminated) and various editions have over/under-emphasized it. Still, the main focus of the game is to do things you couldn't do (easily) in the real world (such as exploring old ruins, fighting monsters, and acquiring treasure). Its escapism, and there's nothing wrong with that!