I'm not the parent poster, but to me it's not about C# in particular, but that there is basically no other well-supported gamedev framework in a language that's 1) statically typed, 2) garbage-collected for safety and productivity, 3) has decent performance, and 4) has good tooling/IDEs. JavaScript, C++, and Python/GDScript are all missing at least one of those. Java could possibly fill the gap, but it's still clunkier to program in than C#, and no Java game framework has much traction.
I'd be ecstatic if there were something checking those boxes that wasn't owned by Microsoft.
However I would point out that before Unity took off, jMonkeyEngine had a good following.
And since reference counting is also garbage collection from CS point of view, I would add Swift with the game development kits, however that is pretty much Apple only, thus not really the same league.
Microsoft unfortunately since XNA, the whole DirectX team has been anti .NET, they didn't even provided Windows Runtime Components for .NET Native, even though it would have been relatively easy to do so, as it is based on COM (plus some extras).
The moment key people responsible for XNA left the building, it was done.
jMonkeyEngine is on my list of things I want to do a project in. I hope it continues to get support. It might be better for 3D than Godot (based on hearsay anyway)
I'd be ecstatic if there were something checking those boxes that wasn't owned by Microsoft.