Your argument has the same structure as "There is no rational reason to drive a car. The most I could gain would be to get where I'm going faster, but the down

Your argument has the same structure as "There is no rational reason to drive a car. The most I could gain would be to get where I'm going faster, but the downside is unlimited (I could die in a car crash)." Just because a bet has unlimited downside and limited upside, that doesn't mean it doesn't have positive expected value, and would not be made by rational actor.