The amount of work needed to write a secure implementation of RSA (you would surely need a library like gmp to handle big numbers) is not worth my time, I would gladly trade that against some download/compilation time.

The same goes for efficient map/reduce framework or even a key/value database (what's wrong with using BerkleyDB/SQLite instead of writing your own storage format?).

I need to serialize user-supplied data ? I'm gonna use a library that will handle edge cases I can't think about.

But I'm not gonna use SpringBoot only for its logging facility, or Django only for its ORM, or a BNF parser generator to parse "hello world".