Because 99% of web dev code is the same stuff and 99% of the time it doesn't need to be rewritten. You don't need your own implementation of URL rules, user authentication and roles, testing suites, database driver or ORM, request parsing, csrf protection, session handling....

I can buy a book on Django/Flask/Symfony/Spring/Express. I can't buy a book about your homebrew 30k LOC "non-framework" which does all the same things plus an additional customized CRUD layer.

> Nobody sees any other kind of application written by a previous programmer and cries "oh no! I am entirely at the mercy of this previous programmer! If only they had used one of the dozen competing frameworks this tragedy could have been avoided."

If a large majority of the code you've been given to maintain could instead be handled by well known, well documented project with thousands of eyes fixing bugs and adding features, you would probably say the same thing.

Imagine inheriting a java codebase of a simple POS system with a UI. Except instead of using Swing or AWT or I don't know what else, the entire UI is custom written in OpenGL. And instead of using any of the standard sorting or ArrayList implementation, they write their own, which are similar, but not identical (and often have 0 tests).