If you hack your system, you can play as them, or via the Challenge Tower (Goro, consoles) (Shao Kahn, VITA).
Other than that, TBH, I'm completely fine with them not being playable.
They have entire move-sets, and can be seen if it's hacked because it was probably left there from when they coded the game, but they were never meant to be playable characters.
Releasing them as DLC would also mean that people would constantly pick them to boost their stats online, given the way that the MK Kommunity is these days about being sore losers.