It seems like most of the nice-sounding ones are proprietary.

That's pretty standard. Most FOSS projects don't have corporations feeding them 100's of thousands of dollars. Even when they do, well people still say gimp is far worse than ps. Blender is one of the rare complex projects that can compete with proprietary alternatives.

And any ideas why this is an underdeveloped area for open source?

My best guess is that it's really expensive and time consuming. I'd be surprised if those really good proprietary models didn't cost $100k+ just for training.