Setting up your own git remote takes minutes. Github is just a pretty interface with bells and whistles. I think more people should go self hosted. It's cheaper, and none of the features are truly needed.
I think ease of contribution and discovery shouldn't be underestimated. I find it hard to believe that our little project [1] would have had 90+ contributors, of whom ~8 made very substantial contributions, if we'd self-hosted it. The project would have been far lesser without those contributions, and I probably would have abandoned it out of lack of interest.
That said, most other projects I see around have much more uneven contribution levels, with typically 100:1 ratios from the core developer to the second biggest contributor. Maybe we're just weird.
[1]: https://github.com/tridactyl/tridactyl/graphs/contributors
Another sad fact of life nowadays is that github issues reduce friction but a lot of "hardcore" developers prefer email or some other offsite system that has lots of friction to people getting involved(i see this a lot in gitlab/bitbucket/sourcehut hosted projects). which means an open source noob is going to look at another project instead of getting involved with yours(again not an issue with you). for better or worse github is king here.
most open source projects don't bother and tend to rely on interest levels to keep them afloat. this works for the popular ones but tends to sink niche projects
Maybe none of the Github features are useful to you in how you use it, but I find Github the most productive way to work with a distributed dev team that I've ever found by a large margin. It's an amazing system for productivity of large dev teams and a dream to work with if it fits your use case.
The biggest feature for me is the whole Pull Request flow with integration between filing the original issue, the code changes to fix it with inline code reviews and tracking everything through to a release all with a few clicks. And all of it is in a format you can link to in an email or drop into a chat, even referencing the specific lines of the diff you care about. It's really an amazing system that most of us take for granted.
Different people have different needs. And for a lot of people, git itself is just the (excellent) engine but really they want a whole car.
Add to that the user permission system which Bitbucket like services provide out of the box (I don't know about GitHub), for example a certain team in your company can access certain repositories, interns can access other repositories etc.
Add to that when you want one of your customers to have a read-only access to a repository, so you not only have to manage user permissions, but your whole IT structure needs to provide outside access to a certain part of your network, in a secure way.
This is achievable but it certainly wouldn't take just minutes. Services like Github and Bitbucket provide all these features for a reasonable price, and they have dedicated sysadmins working on providing a secure service 24/7, who would do a better job than a lot of companies can do with their 1-2 person team doing sysadmin things in their free time, besides their main daily workload.