Software quality in macOS was important back when they were trying to get people to switch from Windows-based PCs to Macs. Nowadays, most people who were going to switch have already switched, so Apple has no incentive to keep up the same level of software quality anymore. They just have to keep people locked into their ecosystem (with iPhone etc.) enough that the barrier to switch out again is high enough.
There is no reason for Apple to improve macOS, since doing so won’t make anyone switch to Macs who hasn’t already switched, and not improving macOS won’t make anyone upset enough to switch back. Ergo, Apple leaves macOS to stagnate, and they will keep macOS at this bad-but-not-horrible-enough-to-switch level for the foreseeable future.
That’s my theory, anyway.