Why would you leave bugs in the code? Why would you treat your software as unimportant? It's about self respect and recognition by others when you produce quality code, and undermining your efforts makes us stray from our Bell Labs predecessors.
The reason is simple - there isn't enough time in the day to both push out the required features and fix all the bugs. We prioritize features because that's what we believe increases our value. Whether or not that's actually true is a matter of opinion.
Our predecessors were expected to write code more slowly. They could take a year to make a small business app that a dev is expected to push out in 4 weeks now. And all credit to today's devs, we hit those deadlines. We achieve amazing things in very little time. But in our push to be as productive as possible we have largely forgotten that the most fundamental skill in software development - thinking about how to solve a problem well - hasn't changed at all.
We believe that using a new framework, or buying a faster computer, or typing on a different keyboard, will make us faster, more productive developers, and in our effort to compress the amount of time it takes to build something we have also compressed the amount of time we spend thinking about it. That's why we leave bugs in code.
A "small business app" would be far too expensive to build to "NASA" Standards, and furthermore, it would be wholly unnecessary to do so. Think opportunity cost.
I have worked in places where, basically, there was so much money pouring in that the drive to actually ship anything had all but disappeared. Thus the 'axis of perfection' took over: Build the perfect shining crystalline entity, no matter the cost. Rebuild your own tools, even where good, liberally licensed FOSS alternatives already exist. Etc.
I found the latter environments too frustrating to cope with, as I found "making code look nicer" did not lead to an existentially satisfying week.