Because LLMs are just stochastic parrots and don't do any thinking.
The same arguments are always brought up, often short pithy one-liners without much clarification. It seems silly that despite this argument first emerging when LLM's could barely write functional code, now that LLM's have reached gold-medal performance on the IMO, it is still being made with little interrogation into its potential faults, or clarification on the precise boundary of intelligence LLM's will never be able to cross.
>Proof: I took a convex optimization paper with a clean open problem in it and asked gpt-5-pro to work on it. It proved a better bound than what is in the paper, and I checked the proof it's correct.
https://x.com/SebastienBubeck/status/1958198661139009862
(please excuse the x link)