All-in is not a wise decision. Even though we have a 90% chance, there is still a 10% chance that we can lose. Of course, we will be happy if we get that 90%. But how if we get 10%. Simply put, when we are playing poker, we have a Straight card on hands, we are 90% sure, if we will win. However, the reality is that our opponent has a flush on hand. that maybe only 10% chance of he get that flush, but our confidence in going All-in with a 90% chance of winning just disappears, because the luck is not on our side.
You are right, and 10% is still a large percentage for one to risk all their money, I would say one shouldn't even do it if they have a 98% chance of winning because there is still a 2% chance of losing and if you are unlucky, the bet will go against you, it's gambling at the end of the day, and you can't expect to win a bet even with 99.9% winning odds in your favour as long as you are unlucky. So, it's better to only risk what's reasonable for us to risk.
When you risk only a small percentage of your total bankroll and lose it, you will still have some hope that you have most of your bankroll safe and you might make that money back, but even if you don't, you at least get to gamble for a while instead of starting, making one bet, and losing everything in it.