The reason it cannot play well is because it has very little 'experience' (training data) with it. It's been trained on 'what the game is', it has not been trained how to win.
You can think of it a bit like driving. You can know what driving is, but it doesn't make you a driver if you've never driven before.
You can ask a genius who's never played before to play tic tac toe with you, tell it the rules, they will likely not win on the first attempt or play optimally. This doesn't mean that person isn't a genius.
You said humans are able to 'learn' to play it rapidly. So is GPT, in training mode, it can process a million games in seconds, where a human can't.
The problem here, is it simply has no experience.
If I told you every time you played tic tac toe against me, you would forget all your experience the next time we played, would you play optimally?