Because they don't care. The vast majority of programmers are people for whom practicality triumphs over aesthetics. If you ever tried to polish something endlessly you know it's a lot of work. Most programmers see it as useless work.

As an open-source developer, I decided to change this state of affairs and create an ecosystem of open-source apps that focuses on user experience and aesthetics while not compromising power users. I've been doing this with my note-taking app Notes[1].

[1] https://github.com/nuttyartist/notes