Well, I think you look at it in a wrong way or don't understand the issue. You see, in FATE's you have a 10+ people slashing and smashing monsters till those drop, or doing some other minor task.
In dungeons you can have story elements. You also have a limited group which has to tactically figure out how to beat the enemy (Copperbell had a great final boss in that regard IMO). A Dungeon has structure, rooms to explore, sometimes dialogue or other things to read, many different/varied monsters to slay (sometimes you have to find an unconventional way to do that) and rooms with loot that such monsters guard (which are optional - that's a good thing). The difference is the level of complexity, freedom and overall stuff to do.
Dungeons are designed so, that each one is a different/unique experience.
With FATE's most of the time you wait for a monster encounter to spawn and then you gang up on those monsters with 10+ other people in a chaotic, loud mess.
Wait, then hack'n'slash. Wait, then hack'n'slash. Wait, then hack'n'slash. Wait, then hack'n'slash.
You don't even have to be in a group to participate. You don't even talk with people.
