I don’t even understand why people like GitHub so much, its source management sucks.

I agree with this part.

GitHub bringing everything into one platform is atypical and obviously done for the goal of centralization.

Perhaps this is part of the answer to why people like github. Unlike you, most people love all-in-one tools. I once suggested a bunch of offline tools to use with git, with much better user experience than github. The other person was like, “Yeah, no! I don’t want to learn that many tools”.

Look for ways to do things separately and you will find much better tools.

The advantage of a centralized app is that all the services you mentioned are integrated well with each other. The distinct and often offline tools often have poor integration with each other. This is harder to achieve in such tools, compared to centralized hosts. The minimum you need to start with is a bunch of standards for all these tools to follow, so that interoperability is possible later.