Why do open source software UI and icons look so ugly and old-fashioned?

Come on, let's get real. Most of the open source apps we have out there are ugly, or at least, old fashioned. I mean, look at LibreOffice 6.0.0, it is a powerful, ungraceful tool.

LibreOffice in 2017-2018, are you from 2004?

I had my first "real" Linux experience when I was a teenager and installed Kademar (I think it was 2008-2009); I didn't care so much about UI and beautiful crystal clear designs.
But, after the Windows 7 release I realized that Linux was ugly. I moved from Kademar to Ubuntu 9 point something. Ubuntu clung to that detailed and colorful style that characterizes almost every Linux app icon.

Kademar in 2008

Why there are plenty of good open source apps but nobody seems to care about UI (User Interface)? Is it too difficult to implement concepts like flat design? I don't think so.
It took a very long time since Unity and Gnome 3 appeared. But even today, there are many things to polish.

Shit, look at the Folder icons, the Title bar buttons, and the upper panel. And still I love Linux.

Now, look at Windows. It's clean, beauteous. Or, at least, more clean and beauteous than Ubuntu.

The exceptions are the big players: Firefox, Thunderbird, VLC (although its design looks old fashioned, it is not ugly), Atom, Inkscape, Gimp, Shotcut, Chromium, etc.

It seems like a bunch of nerds (I'm a bit nerd) are developing great software, but they don't care about design, only matters functionality. Yes, that's true for most of professional-enthusiastic users (including myself), but scares off the mainstream user.

No. Seriously, look at what happens when you search for games in the Microsoft Store VS. Ubuntu Software Center. Beautiful and amazing games and menu' icons they are.

Of course, right now Ubuntu and Kubuntu are beautiful enough to fight Windows, and we have other pretty alternatives like Solus, Antergos, ElementaryOS, Ubuntu Budgie and Nitrux, just to mention a few. But every time a look for open source apps (Windows, Linux and/or MacOS), look at details (like icons or menus) or dive into any distro Software Center, my eyes hurt. We must forget about those 1997-2008 design ideas, they ruin open source software.

That said, I love open source apps and Linux distros and developers who make them possible.