It also isn't exactly single threaded. It calls fork when it wants to persist data to disk, which is functionally similar to starting up a thread to do disk IO.
Also, it might now have any benefit for you. Imagine a certain key is particularly hot. Having one multithreaded redis process handling access to it might speed things up. Running multiple sharded redis processes won't, since only one of them will have that key.
It would make a little difference if there was a single hot key, but that's a bit unusual. Typically there's some subset of keys that are hot, and you can get them to hash across instances. People also tend to cache those values on the clients, as banging on redis constantly is a waste.
It's pretty common when using a single Redis stream as a lightweight Kafka.