"People seem to gain something by trying to be the person hardest and last to impress. "

It's not considered positive to be impressed easily and often, that's similar to how a child acts, which equates with immature for most people. Also, if you are hard to impress then you must have experienced more than those easily impressed. For some it's a way to appear superior with just a simple opinion. Basically arrogant assholes.

The alternative is cynicism but then again that's also sometimes used to seem elite.