Blender itself is also over 20 years old. And it struggled a lot even when opt source until several things came together at once. A mix of a UX overhaul, autodesk pissing off the community, and outreach yielding fruit as corporations experimented with adoption.

I'm not sure if we had that perfect storm in game engines yet. Unity fumbled big time, but Godot wasn't quite mature enough to fully take advantage of that opportunity.