[zmb_]

It makes sense to separate the problem into multiple different protocols, because that gives you re-usability and greatly reduces the complexity of implementation. Can you imagine the amount of effort if every application protocol defined its own mechanisms for retransmissions, congestion control, security, etc.?

It also makes it possible to swap out parts more easily. For example, to put SSL under HTTP. Or QUIC under HTTP/2 when people realized TCP is not a good fit for it. You could, if you wanted to, run HTTP over UDP. Although you'd quickly realize you actually want many things that TCP and TLS give you for free, so you'd have to start re-implementing the same functionality on top of UDP.