Disclaimer: I'm a KDE developer though not super active.

Regarding paid developers, it used to be a few people working for Suse on KDE vs a lot of people working for Red Hat on Gnome. That's not all there is to it because team Gnome also had to maintain GTK and team KDE didn't have to maintain Qt. Gnome had the advantage of having people paid to work on low-level services such as libxml, libdbus and such. AFAIK, today no desktop has many developers paid by distros. You can see it for example on GTK being maintained by like one guy.

Reportedly, external people and companies find KDE is easier to work with than Gnome, and that probably at least played into Valve selecting KDE. Valve's support led to (among other things) KDE having a really really good compositor / window manager. Highly reliable multi-screen support, fractional scaling, high dynamic range, variable refresh rate, frame scheduling for minimum latency (hell yeah), dimming with and without backlight control... a lot of good stuff.