Part of the enjoyment for some people is getting to the max level, Fanest. And for some of those people - like myself - there is a sense of actual accomplishment at then getting to play around with the max leveled character. I would much rather have at least a dungeon or area or two to play around in once I hit max level rather than never hit that or hit it right before the end of the game.
It's not about "being a God" - which you were the first person to mention - it's about planning ahead for certain builds and then getting to play with the final product. It's like putting together a puzzle. Your stance seems to me like someone saying that we shouldn't get to enjoy looking at and maybe placing the puzzle somewhere nice once it's done because we should be concentrating on the act of fitting the puzzle pieces together. Fitting the pieces together can be fun, but that's not why I do puzzles.
Personally, I don't see why you're so upset about the fact that we enjoy a different style of play than you. For me, dungeons aren't just about leveling up, they're about exploring, testing my builds and taking on the challenge of different enemies. Loot and xp are just bonuses that further enhance my ability to explore, test, and challenge.
As for dungeon's having poor stories, that's a failure on the part of the developers, not because "Dungeons are just supposed to be for items & leveling up". Frankly, if that's all there was to dungeons, I'd never enter them. The Divinity series has oodles of examples of dungeons that are much more intricate and story based than just Diablo style dugeon crawlers.
At the end of the day, though, each of us enjoys games for different reasons and play in different ways. That doesn't make any of us "right" or "wrong", it just means we have different play styles. Thankfully, they're including the editor, so whichever side the game isn't made for can just alter it so that it is.
