Because GNOME is the only DE with some potential and by not having 2 or 3 simple optional features aren’t getting more traction.
But everyone has different requirements and my “2 or 3 simple optional features” that are missing are completely different than what you think is missing. I couldn’t care less about desktop icons or system trays. I even prefer not having a system tray, as this functionality should be provided via notifications and regular application shortcuts in my opinion.
But in the end, a software project only has a limited amount of resources available and developers have to decide where they want to focus on. GNOME chose not to focus on desktop icons:
GNOME had icons, v3.28 discontinued them
Because the code was “old and unmaintained” and probably no one was willing to modernise and maintain it. Desktop icons were already disabled by default before 3.28, so they didn’t “re-invent” this feature with the removal of the code in Nautilus.
Using other DE doesn’t make much sense as you’ll inevitable run in GTK and parts of GNOME and having to mix and match to get a working desktop experience.
I use GNOME and KDE and use the same applications (as Flatpaks) on both desktops: I use GNOME Calculator on KDE, because I dislike both KDE calculators, and I use Ark on GNOME with a Nautilus script, as File Roller doesn’t allow me to set the compression ratio (I need to create zip files with 0 compression for modding games). So for me it has become the norm to mix applications created with different toolkits. Thanks to Flatpak I still have a “clean” base system though.
Btw. I am getting tired of these re-occurring complaints that GNOME works differently than other desktops. I am not constantly complaining about what features KDE is, in my opinion, missing all the time either (e. g. dynamic workspaces, same wallpaper and desktop configuration across all existing and new monitors, online account integration, command line config tool, etc.), instead I accept that this is how it is at the moment and either use KDE the way it is (like I do on my desktop PC) or use something that better suits my needs (like I do on all my laptops).