Because most software devs don’t stick around long enough to write software that can stand the test of time. It requires a few things: like don’t use libraries unless they are well funded (includes frameworks) or have already stood the test of time (like bootstrap for the FE or React, etc); don’t hide logic deep in the code, put logic as high level as possible (like in controllers, which is the exact opposite of short-lived software; and many other little things.

I have several projects running in prod, at several companies, for over a decade. They are easy to maintain, easy to extend and build on, and easy to understand.