Because OpenAI has the hype and delivers the pop, not AMD. An AMD investment in OpenAI would probably have had a similar effect on AMD, not an equivalent one on OpenAI. (Either way it's Ooh AMD is getting seriously into AI, awesome, suddenly it seems worth 100x whatever it was before!)

Replace AMD & OpenAI with some crypto company and Trump, hypothetically the latter doesn't really obviously get any significant benefit from the former's endorsement.