Better for everyone to be using a small handful of battle-tested implementations written by experts than for everyone to roll their own implementation. The latter may mean that people have a better understanding of the component, but it's also pretty much guaranteed to mean the various implementations are buggy. Even very simple protocols are easy to introduce bugs into.
For example, it's pretty easy to write an HTTP/1.0 implementation, but it's also easy to open yourself up to DoS attacks if you do so. If you're writing a server, did you remember to put a limit on how large a request body can be before you shut down the request? Great! Did you remember to do that for the headers too? Limiting request bodies is an obvious thing to do. Limiting the size of headers, not so much. But maybe you thought of that anyway. What about dealing with clients that open lots of connections and veeery sloowly feed chunks of a request? The sockets are still active, but the connections are so slow you can easily exhaust all your resources just tracking sockets (or even run out of file descriptors). And this is just plain HTTP, without even considering interacting with TLS.
For example, it's pretty easy to write an HTTP/1.0 implementation, but it's also easy to open yourself up to DoS attacks if you do so. If you're writing a server, did you remember to put a limit on how large a request body can be before you shut down the request? Great! Did you remember to do that for the headers too? Limiting request bodies is an obvious thing to do. Limiting the size of headers, not so much. But maybe you thought of that anyway. What about dealing with clients that open lots of connections and veeery sloowly feed chunks of a request? The sockets are still active, but the connections are so slow you can easily exhaust all your resources just tracking sockets (or even run out of file descriptors). And this is just plain HTTP, without even considering interacting with TLS.