In a second preimage attack, you start with an input that comes out to a hash, and you're tasked with making a distinct input that comes out to a hash.

In a first preimage attack, you start with only the hash, and have to come up with an input from scratch that comes out to that hash. Even in a first preimage attack, you're extremely unlikely to craft the same input that someone else used to create the hash you were given. By pigeonhole principle, the number of inputs that come out to a shorter hash is extreme.