Because you're reinventing a wheel that doesn't need reinvention, and the most likely thing is that you will neither reinvent it nor pick the best library that an opinionated framework with hundreds of eyeballs has.
> again, match for that, map your url parts, whatever - it doesn't need a 10,000 pound gorilla when it can be done in a line or two of code
Sufficiently large and complex websites will have that need.
> But, if you're building a single product over the course of a year or two, it's not the end of the world to spend a couple weeks rolling your own auth and hook it up to transactional emails and everything else. It's just one small problem to deal with, not major in the grand scheme of things. YMMV
No individual problem is large, but it is objectively a dozen little problems, all with a nontrivial chance to blow up into larger problems.
Code reuse and frameworks exist because, unless you've been doing web development for a long time, you _will_ run into issues that have already been solved.