Not too surprising. It's built into gaming's history to work on and release a game purely woth in house tech. And then submit it to a proprietary console maker for a "seal of quality" (or what used to be that). And then other important tools (especially for artists, but a lot for programmibg) served this same mentality as AAA development grew. Support was more important than anything else as tools became more comple, and these studios weren't used to sharing to begin with.

The phenomenon of open source tools being able to even be considered by professional development is a very recent phenomenon (in the grand scheme of things). But for programmers it's still difficult because console support is so important and still extremely closed down.