Why is collateral needed if I have 2 utxo's that are 6 and 2?

When I want to withdraw from a DEX, sometimes I have to set the collateral in CCVault. I understand this part, but sometimes I have to create utxo’s with 5 and 3, from two utxo’s that are 6 and 2? Why can’t 6 be chosen for the collateral position?