It's called design and thank god everything isn't the same.
Gtk has some nice standard looking buttons for you. You know how those look, and I'm pretty sure you don't want that.
Think to when "UI experts" impose edicts from top down, such as "flat design" or "material". Are you really happy about that?
The fact that we're even looking in awe at this list is amazing. If everything were the same, this wouldn't even be interesting.
This is fun and we need more of it in the world.
Lack of consistent system-wide UI and UX was my biggest gripe with GNU/Linux/X11 back in the day (‘90s, pre-Winamp era, other OSes had most apps with uniform look-and-feel) while I still cared about things like that.