It's because applications can't get critical mass. The desktop itself has a fairly small number of developers, but "everyone" using Linux is interested in it and the Linux distributions sponsor it a little bit (not a lot).
For apps, maybe 5% of those using Linux are interested in a given app? And number of OSS developers relates to number of users because generally some users become developers. Also for apps, fewer of the users know how to code (many developers use only desktop, browser, terminal, and editors, and little else).
But _each_ app can be at least as much work as building the entire desktop. And the distributions can't afford to sponsor many apps.
And on Linux there hasn't been much success with proprietary apps.
So that's why there aren't tons of apps. (In my opinion.) In fact it's sort of amazing how many there are. I'd say most of them have only 1 or maybe 3 core developers though.
The apps with the most developers are cross-platform. (Firefox, LibreOffice)