I have a web music player that I've developed, and while it was never really intended to be used by others, I thought I had generally followed accessibility best practices. After using it for about two years, I realized that I never even implemented keyboard shortcuts. 🫠

Which is to say, one shouldn't assume devs to know what they're doing. At some point, I'm also just a user and I use software like everyone else does, meaning I pick out a path that works for me and then I hardly look left and right from there.
Features not being tested when you don't use them yourself, that happens with any feature. But it's much worse for UI features, because those are difficult to automate tests for. And accessibility is in an even worse spot, because it necessarily opens up a separate path, which is going to be invisible to me as a user, so it gets covered by neither automated tests nor by me just using the software.

I have to go out of my way to test accessibility, which means I have to be aware that a change I'm making might introduce a regression. That's genuinely how lots of amateur developers work, which is probably the best explanation why accessibility support is often so amateur-ish...