Ryzen desktop chips use the same compute chiplets as server (server has a long tail for adoption and a steady need for replacement parts), so they have a large supply of chiplets that don't meet server requirements but can be downclocked or given more voltage for desktop. This also fulfills the low end desktop market, so they don't have to produce lower end chips on more expensive nodes. There's also a lot of AM4 platforms that can get a new lease on life with a drop in Zen3 replacement.

Then you also have supply from the laptop side with similar issues (don't meet voltage requirements for efficiency), which is where the APUs come from.