Bribery
All right, all right; I can't very well try to witter on endlessly about why dungeons are so great without trying to appeal to the grubby, greedy souls of the players. We've no secrets from each other; let's be open about our motives. You want your characters to be rich and powerful. Granted, you might actually want to do some of this 'role-playing' we've heard so much about as well. Takes all sorts, I suppose -- but how much role-play you get is really your affair; you get as much as you give.
As you all know by now, if you want treasure you have to look for it. Where would you rather look? A forest? A plain? Or a multi-room complex where potential treasure-acquisition zones are neatly delineated?
Take your time.
As for experience… outside of a war zone, the dungeon (as I've mentioned, if you've been paying attention) is the best place for concentrated hazard. Outside, what can you expect to encounter? Beasts? They've got plenty of room to retreat if the encounter goes against them. Traps? Nothing beyond the occasional gin-trap or improvised snare. Thieves? Only the occasional thief in a town or city, or a few disparate bands of desperate bandits. Nope; if you want a ruck without a criminal record, you have to dig for it.
That's not to say, of course, that battle, glory and treasure can't be had on the surface; it's just that the lack of constraint means any potential enemies have far greater range in which to roam. How many days of game time are you prepared to put up with this?
GM: You pitch camp for the night on the moors. The weather's cold and many people have died of exposure out here, but thanks to your tiny hut spell that won't be a problem.
PLAYER 1: I take first watch.
PLAYER 2: I take second watch.
PLAYER 3: I take last watch.
GM: Okay. Player 1; on your watch: (rolls dice)… nothing happens.
PLAYER 2: Here it comes…
GM: Player 2! On your watch: (rolls dice)… nothing happens. Player 3:
PLAYER 3: Always happens to me.
GM: (rolls dice) … nothing happens. Your rest is completely undisturbed.
PLAYER 1: We break camp and continue looking for the goblin warband.
GM: Okay. On the first hour you find no trace of them, and… encounter nothing…
OMNES: Zzzzzzzz…
Sure, there's the occasional lull in the dungeon, but they're usually tense, fragile periods of quiet. You know that danger isn't far away.