My impression is there's a definite shortage of GPUs, and if OpenAI is more reliable it's because they have fewer customers relative to the number of GPUs they have. I don't think Google is handing out 429s because they are worried about overspending; I think it's because they literally cannot serve the requests.

This sounds very plausible. OpenAI has hoarded 40% of world's RAM supply, which they likely have no use for other than to starve competition. They (or other competitors) could be utilizing the same strategy for other hardware.

Which is worrying, because if this continues, and if Google, who has GCP is struggling to serve requests, there's no telling what's going to happen with services like Hetzner etc.