To me, a corporation cannot maintain quality code because requirements are ill defined, and there is no “done” state. With those two conditions present, unable to be changed, it’s not possible to form a coherent codebase. Those who try will make things worse, because their abstractions won’t fit in a year or two.
This is exactly the “messy code” people then leave behind. Bad code can come about for other reasons too, of course, but this is one of the more annoying reasons, because someone wrote it with self-righteousness, as if they were the only people to truly SEE the problem. Sigh.
It’s fine, this is how enterprise works. You can learn to navigate and make a living from it. You MUST internalize and accept that it is NOT the same as maintaining code for an open source library or whatever people think it’s going to be.