OpenAI is worth $800B because that’s what the last funding round valued it at. It does not mean that >50% of existing shareholders will agree to sell for that amount. If a serious buyer does emerge they will likely hold out for 2x that, maybe more.
Not to mention that the US would have to approve the sale to another country, which they are unlikely to do.
Or 2x less. 800B valued is not 800B invested. And it wouldn't be the first time investors sell at a loss if sentiment declines.
OpenAI is worth the discounted value of their future free cash flow. It's not clear at this point how much free cash flow they will ever generate, if any.