Well, most design people also have side projects, be it a Tumblr page or an endless supply of drawings that they just do to improve their drawing skills.

But that's different from helping an (OSS) application look good. For that, you have to make tons of mockups, iterate again and again, until you have something that looks good and consistent across the entire application. Ideally, you'd also want at least one other person to bounce ideas back and forth, and have an opinion about what you're doing.

This is real work, which requires a lot of commitment upfront. The equivalent of requiring a programmer to lay out the entire architecture before they write a single line of code. That's also something that mainly happens in a corporate environment, whereas for hobby projects it tends to lead to frustration.