Because people fail to understand the probabilities and like to think of the possibility to "hack the system" and find an ultimate winning strategy. You're absolutely right, the chances of losing while placing a single bet and with Martingale are the same.
It's pure psychology of self-consolation, that does the trick. When placing a single bet a gambler thinks "I might win, or I might lose, it's a 50/50 chance" (with respective odds, of course).
But with Martingale the gamblers are often deluded by the thinking that with each bet the chances of winning increase. They go like "I've already lost 6 times in a row, I will definitely win in the next round". While, in fact, the chances are still the same and they can keep losing.
Your logic makes sense and I believe, a lot of gamblers have that kind of perspective.
This is why some like to use martingale system because of that perception.
Anyway, in my opinion, if the game is luck-based, whether you use martingale or one-time bet, still your chance of losing is always there.
Don't think that you have high chance of winning when you practice either one of these strategies.