Beowolf wasn't fighting Grendel to steal his stuff, Arthur wasn't fighting Modred to take his stuff and Frodo didn't go to Mount Doom to steal anything, but to actually destroy a powerful artefact.
It strikes me that core D&D posits that "good vs. evil" is one of main themes of the game. Part of the reason we have alignments, even. "Getting their stuff" is just standard wartime pillaging. Of course, when you slay the bad guys, you put their knowledge to the use of the Good guys.
In other words, "beating up things to take their stuff away from them" isn't the only default motivation for characters that the rules assume. The rules assume that they'll want to save the princess, stop the advancing orc horde, kill the necromancer, and overthrow the corrupt empire because they are Good, not because they want their stuff.
Of course, there's a large element of simple exploration, too, wherein it's a bit more mercenary. The dungeon exists for you to go into it and earn fame and fortune braving it's dangers. It's as heroic as, say, the journey of Columbus or the other Conquistordores -- or even Indiana Jones. Going into some dangerous unknown territory for the good of your homeland (and, of course, yourself). That's Adventurous, but it's not entirely Heroic.
But then, in the course of that adventure, they perform great feats of magnificent daring and courage that are definately beyond most mortal ability...thus shaping up to be Heroic, though in a different way.
They fight evil and perform great deeds. This is *very* heroic material, stuff you'll find examples of both in legend and history. To consider otherwise is basically to limit the definition of "heroic" to something more artificially narrow (like "only normal people can be heroic!" is artificially narrow).