And many people use MechJeb, kOS, kRPC, Kramax Autopilot, Throttle Controlled Avionics, etc.
Because supply and demand.
KSP is a game engine providing the possibility of automated flight, and this possibility is demanded by many users.
Alarm Clock, Infernal Robotics, RemoteTech, and KIS also were not from the very beginning.
But later they had been added either to stock, or as DLC.
So, the flight automation is an important part of the KSP flight system, which somebody uses, somebody doesn't.
Obviously, they won't be implemented in the stock game because the project is finalized, but this does change nothing.