I believe for many companies, developers work on giant codebases with many hundred thousands or even millions of lines of code.

With such large codebase you have no control over any system. Because control is split between groups of devs.

If you want to refactor a single subsystem it would take coordination of all groups working on that part and will halt development, probably for months. But first you have to convince all the management people, that refactor is needed, that on itself could take eternity.

So instead you patch it on your end and call it a day.

Half of the linux ecosystem is personal projects.
Linux itself started as

just a hobby, won’t be big and professional like gnu

It’s not useless as you can learn from it.

From interview: it started as a research project. The author wanted a distribution that uses the least system resources with maximum performance.
He started with archlinux, moved on to gentoo and to go even deeper - found the infamous “linux from scratch” and started to shape his own distro.