Obvious political reasons and implications aside, a clear quality gap opened up late last year when Opus 4.5 was released vs. GPT-5. Opus was obviously and demonstrably superior to any GPT-5 tier. The release of GPT-5.2 didn't improve matters, and then Opus 4.6 widened the gap further. Right now talking to GPT-5.2 Pro is 10x slower than chatting with Opus 4.6 and the output returned is, nevertheless, generally lower quality and more "sloppy."
What I'm getting at is that this could be, in part, because Claude is genuinely better at this point in time.