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.