I get the reasoning why there is only a single port for http connections, but why was the number chosen to be 80? Why not 76, or 111 or 2? What is the reason behind the number 80? (Or why 443 for https? Or 21 for ftp?)
Tim Berners Lee (aka TBL) choose port 80 in the early 1990s; at one time, TCP port 2784 was used, but TBL deprecated it in favor of using port 80.
TBL came up with ENQUIRE, which was the building block for hypertext in 1980, but it took almost a decade for the protocol to mature. One might speculate that 80 is a reference to 1980, but that's pure speculation AFAICT.
There is no real reason we have to choose port 80, other than the need to have a well-known port, and that IANA standardized on port 80 so long ago.