In most of the exchanges, a transaction must have 3-6 comfirmation before your transaction can be consider deposit. They don't release Bitcoin you deposit until that comfirmation is reach before they allow you to do anything with that Bitcoin, you may be allow to trade it with one comfirmation but you can't withdraw it unless that comfirmation is complete. Why?
Second question, is a transaction with 1 confirmation reversible? What are the chances of 1 confirmation Bitcoin reversible?
Well under normal circumstances no, a transaction is not reversible. But this is done to avoid loss in case a pool decides to act up.
It's just that as the number of confirmations increase, the probability of a transaction getting reversed goes down.
Keep in mind that the 2 biggest mining pools control a good chunk of the hashrate. So exchanges would rather just wait for 6.
And also, the 6 confirmations concept has been around for the longest of time that crypto users just got used to it as the norm and no one bats an eye.