I'd say it's because Godot has only recently started gaining traction (probably around it's 3.0 release) and still doesn't have huge tutorial/asset community that Unity and Unreal have. Plus while 3.0 really helped how Godot renders 3D if you already know Unity/Unreal, why bother switch?

I only dabble in game dev, but a few of my friends are far more interested in it. Whenever I've asked them to consider switching they're pretty entrenched in the Unreal or Unity and don't see convincing arguments yet.