The leading 3d gen techniques are latent diffusion models that work off voxelized intermediate representations of objects. When you convert these into meshes, very often they will have poor topology. Topology is important if you need to modify these meshes or animate them. Bad topology and excess verts is also bad if you're building assets for a game.

There's still more refinement of existing techniques that need to be done before AI generated 3d models match what humans produce.