It's much simpler to develop complex libraries outside of the stdlib. And given Rust's strong stability guarantees, it can't stabilize any APIs until they're finalized anyway. So development of new APIs often happens outside the stdlib and then if a third party library proves itself and can commit to a completely stable API then it might be uplifted into the stdlib, so long as there's a very strong case to do so.

I should also add that most applications won't even be using the TCP/UDP primitives directly. They'll likely be using a third party library. Rust's stdlib is intentionally minimal and lacking a lot of higher level features that are practically necessary for many applications.