One way to look at it: collision resistance prevents more attacks than mere 2nd preimage resistance, so that is a "higher" barrier. As you say, a certain type of attack would be to arbitrarily pick the first message and then try to find a collision, but that isn't the only conceivable attack. You might be able to run an attack that would generate message pairs that would have a higher likelihood of defeating a particular hash, if the hash had 2nd preimage but not collision resistance.

EDIT: I agree with sibling comments.