I am sure if you boil it all down it comes down to money in some way or another. Not enough funding or efforts or something along those lines.
A lot of people play GTA games on pc, so I don't see any reason why the devs should failed to do their job well.
I think PC games require a lot of tuning because they have different configuration to test. So it's better to wait in most of the cases.
Those sound like a bunch of excuses. If they are not up to the job, then maybe they should outsource the work to people who can.