The same way those in the smash games meet up for a new franchise, I wager. Because someone wanted them to.
Somehow, yes. but see things as those more as a joke though. I've also made a comic where Mario and Link explore planet Zebes with Samus, because it's fun.
Also, despite I made her as a Terraria OC, her restriction is not in Terraria only, as she also appeared in Skyrim too.