That was my thought too, until I realised the only correct response for the AI would be to also flip a coin. I'd prefer to take the million rather than a 50% chance to beat the AI.
I'm a one-boxer because $1,000,000 is a lot of money to gamble that I would be the (first? only? one of the few?) person who beats the system. I also wouldn't take on Kasparov in a chess match or John McEnroe in a tennis match at even odds; I definitely wouldn't put $1,000,000 on the line to potentially increase my winnings to $1,001,000.