We've managed to make the entire corpus of open source software but the thing that's a "Hard Problem" that nobody can find a way to do is making the icons look good?

It's almost like it's not a technical challenge, it's that getting good looking icons would require a unified userbase, and Apple has that but Linux does not.

Linux these days is almost exclusively developed by companies with combined value exceeding Apple's several-fold.

Finding money and designers is not really a hard problem.