Simply because the stock isn't actually worth 400.
A few players made a smart play on wallstreetbets, and they got lucky when the hedge funds were stubborn enough to keep going deep into the red while the subreddit gained 3m new subscribers and was able to keep squeezing them.
But buying in at 400 today seems like idiocy. It would be akin to joining a pyramid scheme right at the end. Maybe you are not last, but it's likely.