Actually you can connect multiple keyboards/mice to a pc, as long as you use USB/Bluetooth. By default though the operating system doesn't distinguish between them. You have to use 3rd party software for that.
Adding 2 player split-screen to pc Terraria is doable but not trivial, and effectively doubles the memory and performance requirements. Basically you run two instances of the game simultaniously. There was a mod that did something like this (don't know if it still works, I never used it myself). It is unlikely though that Re-Logic will add this anytime soon, if ever. They probably rather spend their limited developer resources on other things, that benefit more people. After all, only a small subset of the pc users will ever use split-screen if that option exists.
Split-screen is more a console thing. Consoles have a long history of supporting it, whenever possible. It also tends to work better there as the console is usually hooked up to the biggest screen in the house.