Because worse is better and if you try to be everything you (don't) get OSI. Also because everyone loves serial ports and dialup modems and TCP just emulates a

Because worse is better and if you try to be everything you (don't) get OSI. Also because everyone loves serial ports and dialup modems and TCP just emulates a serial port over packet network. And it wasn't supposed to be one-size-fits-all, rather «let us have telnet while we figuring out the rest» but then ossification happend.