Two things:

1. Desktop requires mature CPUs (large out-of-order designs with high IPC) and there just aren't really any of those yet. They're starting to arrive (e.g. XiangShan which is even open source!) but as far as I know there isn't a single chip available to buy that's faster than a Raspberry Pi 4.

2. Microcontrollers can get away with only the basic instruction set (add, multiply, load, store etc.) but for high performance you need a ton of extensions that are considered standard. x86 and ARM have had decades to build them up but in RISC-V a lot of them are only recently ratified (e.g. Vector) or still in the process of being defined.

I would say we might see cheap Android phones with RISC-V CPUs in maybe 5 years. Though there's an additional difficulty there in that you need to emulate ARM for games, and I don't think anyone is working on that.