Especially desktop icons are difficult to get right
This doesn’t just affect desktop icons, icons in general suck under Linux. Things have strange behaviors when selected, long names don’t work properly etc.
But most importantly: GNOME doesn’t want to be a traditional (Windows-like) desktop, so why would they implement features that don’t align with their ideas for a desktop experience?
Because GNOME is the only DE with some potential and by not having 2 or 3 simple optional features aren’t getting more traction. I bet half of the KDE users would be glad to use GNOME only if it had desktop icons. 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.
Again, GNOME had icons, v3.28 discontinued them for no other purpose than trying to re-inveting something that worked for a ton of people.