Partly because Anthropic publish most of their system prompts (though not the tools ones which are the most interesting IMO, see https://simonwillison.net/2025/May/25/claude-4-system-prompt...) but mainly because their system prompts are the most interesting of the lot: Anthropic's prompts are longer, they seem to lean on prompting a lot more for guiding their behavior.

Because we don't know the prompts of Google/OpenAI.

I remain rather sceptical about the methods they use to extract these, which boil down to mostly just asking the LLM about it with some tricks to do so against instructions.

And this repo provides no documentation about how they were extracted, which would be useful at least to try to verify them by replication.

as awesome as it is, this is not a definite answer.