int, float, and complex are for different purposes. async and threads paper over each others' weaknesses, instead of fixing the weaknesses at the start. Async itself is an antipattern (technical opinion, so there) but Python uses it because of the hazards and high costs of threads. Chuck Moore figured out 50 years ago to keep the async stuff out of the programmer's way, when he put multitasking into Polyforth, which ran on tiny machines. Python (and Node) still make the programmer deal with it.
If you look at Haskell, Erlang/Elixir, and Go, they all let you write performant sequential code by pushing the async into the runtime where the programmer doesn't have to see it. Python had an opportunity to do the same, but stayed with async and coroutines. What a pain.
Oh, you meant like, why don't Python didn't reimplement the whole interpreter around concurrency instead of using the tools it already had to find solutions to problems?
Well, that question is as old as engineering itself, and it's always a matter of resources, cost, history and knowledge.
Besides, it's weird, like saying we should not have int, float and complex, there should be one way to do it.
Just because those are 3 numbers doesn't mean they don't have each their own specific benefit.