Blockchains depend on small inputs and outputs, and easy verifiability of the computation. AI training is none of that. It's a massively interconnected problem, Google and NVIDIA built traditionalish fast-interconnect supercomputers to do it.
Maybe it's possible to implement it in a decentralized way with not-completely-useless performance, but by then OpenAI and others will be even more ahead. (Sure, it'd be good to have such an implementation, so maybe enthusiasts will do it eventually, but that's mostly for fun.)
Yeah, that's the current state of course, I'm betting on future developments. Neural networks are still in their infancy. Stable diffusion showed everyone how much more powerful open models are (at generating porn /s). Chatgpt is severely limited, cause Openai fear public outrage, litigation and regulation.
Maybe it's possible to implement it in a decentralized way with not-completely-useless performance, but by then OpenAI and others will be even more ahead. (Sure, it'd be good to have such an implementation, so maybe enthusiasts will do it eventually, but that's mostly for fun.)