* https://assets.guitar-pro.com/1.3/images/www/guitar-pro-7/ca...
* https://www.awn.com/sites/default/files/styles/original/publ...
* http://www.comptoir-hardware.com/images/stories/_software/am...
it's not what I use myself, being more of an i3/terminal guy, but it's absolutely what the clients want
I'm sure someone (and, in particular, their designers) think they look nice. But the thing about looks is that they're very subjective. However, if I'm using some OS/desktop combo, that means I find its UI, at the very least, acceptable. And thus any app that uses native UI look & feel on that platform is also acceptable. I can't say that for many custom-themed apps.
For example, out of your links, I would say that 2 are downright ugly, and the third is tolerable. And I'm not sure what "in 2018" has to do with that.