PC releasing at the same time might just rob consoles of sales they could have. While I don't support what they do, this is what I think the most logical reason
In my view this is short sighted. A game with this many systems in it may actually need time to be decently optimised for keyboard and mouse inputs. When GTAV finally released it also carried a bunch of improvements. R* doesn't want to sell games consoles. They don't make any.
It needs to be "optimized" for controller input in exactly the same way. They could've chosen to budget for this "optimization" (whatever that even means) pre-launch.
I think the most likely explanations are 1) larger player base on consoles, 2) Rockstar wants to get the release cash injection ASAP, and 3) staggering platform releases like this prolongs buzz and even leads to a bunch of people buying the game twice.