But that doesn't really matter, because those frameworks have mindshare. If I have a specific issue that I don't know how to solve in Angular or Flask or whatev

But that doesn't really matter, because those frameworks have mindshare. If I have a specific issue that I don't know how to solve in Angular or Flask or whatever, I can search online, and chances are good that someone has run into the exact same issue and solved it. If I'm using someone else's custom-built framework, I have to either consult the original author, which may not be practical, or figure it out myself by digging through the code. For anything that needs to be maintained in the long run, I'd take something written using a popular framework, with all its flaws, over something written from scratch any day.