I'm honestly a bit baffled by the gatekeeping in this thread.
While it's perfectly valid to use one of the big engines, it's fine even for an indie dev to build their own engine on top of frameworks like MonoGame by e.g. implementing a simple ECS or other components they want to use in their dev workflow. Not every game is a AAA game in need of a cutting edge graphics pipeline.
Some of the most accomplished indie developers wrote their own engines from scratch or used low level frameworks to build them.
While it's perfectly valid to use one of the big engines, it's fine even for an indie dev to build their own engine on top of frameworks like MonoGame by e.g. implementing a simple ECS or other components they want to use in their dev workflow. Not every game is a AAA game in need of a cutting edge graphics pipeline.
Some of the most accomplished indie developers wrote their own engines from scratch or used low level frameworks to build them.