Sure, if you are putting in a health check and nothing else, that's fine. When you need to parse headers, negotiate content types, validate maximum lengths on requests, properly log the source of the request (it's not just the connecting IP if there's a load balancer in front of it), parsing request bodies, dealing with range requests, authentication, cookies, providing a good middleware mechanism...

The list goes on. While I'm not a Rust developer, there's probably hundreds of libraries because the problem is structured into a lot of small parts, and frameworks are expected to be able to satisfy the functionality you expect without needing to bypass it.