Even with a 90% chance, it's still a gamble. That extra 10% can bring everything down, and that's where many people get too excited. The confidence of "almost certainty" is what makes people raise their hand, and when it goes wrong, the fall comes hard.
Even as low as 2% can still make you lose your bet so in gambling nothing is certain, no matter how good it may look to you as long as there is a possibility of getting a different outcome than what you expected then it should be considered gambling.
Even an odd of 1.02 or lower can still make you lose your bet and if you do the calculation according to most bookies you’ll see that the chance of losing that bet is lower than 2% and yet bets like that have made people lost their parlay bets - from their potential thousands or hundreds of dollars to nothing just because of a game they thought was certain to win because of the the little probability of it being wrong.