Even if mappers want to use premade assets, it still takes a lot of effort to make a level. Starting from the era of DOOM 1/2, the making of a sophisticated map-pack went from weeks to months and then to years. It's just too much.

I have made no-name maps for BF2, and it was "just" sculping terrain, dropping assets, painting woods, roads and ground terrain.

But when I think about it I think you are right regarding "corridor like" fpses, like Half-Life or Quake, where you need to make the buildings.