UUID is just array of 16 bytes or two 64-bit ints. Generating UUIDv4 is like few lines of code. Is that a big deal? I don't think so.
I think it saves labor and eventual bug hunting to include these in a stdlib. We should not be expected to look up the UUIDv4 spec and make sure you’re following it correctly. This feels like exactly what reasonable encapsulation should be.
You can say this for everything that has built-in support.
16 random bytes is not a valid UUIDv4. I don’t think it needs to be in the standard library, but implementing the spec yourself is also not the right choice for 99% of cases.