Ah, that is until you look at the average salaries for developers versus designers.

Heck, even look at the average salaries of illustrators vs. graphic designers.

It's also MUCH easier for a developer to bootstrap and start a project on their own (be it a app, website, or add-on) than a designer/illustrator. The latter group can start some interesting projects on their own without needing to code but they are GENERALLY less scalable and lucrative.