Because of the NCP protocol which preceded TCP. It used even port numbers for outgoing data and odd port numbers for incoming data. TCP used the same port numbers, but fully duplexed the data into a single port. The well-known incoming port numbers are therefore all odd (initially, the limitation no longer applies to TCP of course). https://en.wikipedia.org/wiki/Network_Control_Program

That was to prevent homosocketuality.

http://www.saildart.org/IMPSER.DOC[SS,SYS]

IGN 3 Illegal Gender (Anita Bryant feature--sockets must be heterosocketual, ie. odd to even and even to odd)

The host is trying to engage us in homosocketuality. Since this is against the laws of God and ARPA, we naturally refuse to consent to it.

http://www.saildart.org/FTP.NCP[S,NET]

gayskt: outstr [asciz/Homosocketuality is prohibited (the Anita Bryant feature)