Erlang makes heavy use of green threads to do this kind of work. You spin up thousands or hundreds of thousands of these. Kernel threads are too heavy weight to do so. And making them lighter weight has tradeoffs too. Erlang makes use of a GC and immutability to make tasks more restart-able; maybe Rust's memory safety features would let you do this sorta kinda, but I don't think it's been really demonstrated fully yet.

Like sure, you could build out some of these features, but they only truly work really well if you extremely commit to the architecture, in my opinion. And the kernel isn't about to do that.