Probably because there's no money in doing it. Not to mention none of the companies you mentioned really do anything innovative they just produce PC compatible clones and rebadge other companies hardware.
EDIT: Heh looks like NTMBK beat me to it by a couple of minutes but yeah ...
Is there even a standardized socket for RISC-V so far? Since if not you're essentially asking why AMI, Asus, and Gigabyte don't sell full RISC-V systems. Do they even sell full ARM systems, as a point of comparison?
This is pretty much a chicken-and-egg scenario. As an end consumer, I don't want to pay much higher prices for a product with lower performance and a smaller Software ecosystem than staying in standard x86. If it was competitively priced against x86 it would make sense, but that is impossible for a low volume, niche product, so they will only sell to power users with a lot of disposable income, which is not enough incentive to get economy of scale working to lower prices and increase availability. As such, is hard to get the snowball rolling downhill.
There are very few commercial RISC-V processors available at the moment, and as far as I am aware, none of them are available "socketed". Now, that's not to say they couldn't just make a motherboard with a soldered on CPU, but that's not really their thing.
The second issue is that there is very little demand and support for them at the moment. It's still very much an experimental thing and in the very early days.
As pointed out by others here, it is early days for RISC-V in DIY. But it is now becoming available in SBC (Single Board Computer) form factor. Have a look at BeagleV:
RISC-V Single Board Computer Designed To Run Linux
"Suitable for edge computing and AI, the board comes with a host of computing characteristics, which allows high flexibility to Linux users —BeagleBoard in collaboration with Seeed has announced that it has brought in Star Five, a RISC-V solutions provider for the development of the BeagleV (pronounced Beagle five) Single Board Computer, providing developers with more freedom and power to innovate and design. It features 8GB RAM with powerful AI performance (3.5T NVDLA, 1T NNE), built-in ISP/NNE, PCle 3.0,1 Gigabit ethernet and a dual-core 64-bit RISC-V CPU with 8GB of LPDDR memory. It also has a dedicated hardware encoder/decoder supporting H.264 4k@60fps, making it a perfect edge computing device with powerful AI capability."