Not all, I'm noticing a trend of including Chromium on the more niche distros like the Fedora spins (I noticed this about 1-2 years ago, the situation may have changed).

Not all. Many include WebKit based browsers (pretty much all except Firefox are, and Firefox is too heavy for many).

I also blame the Mozilla Corporation for that: including WebKit in your software is a piece of cake. So that's what the default browsers of GNOME/KDE/etc do. Gecko not so much (in fact, has there been any effort in this arena, after XULRunner was discontinued?)

Gnome is literally running using Firefox's technology

Please elaborate. I don't know any project embedding Gecko.

For Android at least they're working on https://github.com/mozilla-mobile/android-components/