If you're making a wheel that exactly fits the design requirements you'll have a hard time when does requirements inevitably changes. Making rigid designs instead of plyable is a terrible idea if you're working with web
Nah. Changing your own code is easy. I dare say it also might be the most fun aspect of programming.
Changing someone else's (meaning an external third-party) code can be as easy and fun with the right tooling, but the tooling around supporting that tends to be really bad in most ecosystems. There is a common assumption found around dependencies that their code is theirs and shouldn't be touched except by the hand of the original author.
Not impossible to overcome, of course, but with enough friction that you have to seriously consider if it beats simply writing your own. A little copying is better than a little dependency, but a where there is a lot of copying...
A framework with all the bells and whistles isn't that great when I want a framework with a piano.