Hello there!
There are several uses for the present perfect tense. One of them, the one that appears to be used in the line you referenced, is an action that has been repeated over time with no specific points of reference or time frames and potentially including the present moment. In the second part of his quote, "This wasn't it." he is using the simple past to isolate the time frame reference of the action in the previously stated present perfect. In this quote Mr. Marx is specifically excluding the most recent (or 'present') moment as part of the time frame for the action of having a wonderful time. So, he's using two tenses, one to opaquely imply that he recently (may have) had a wonderful time followed by the zinger that flatly denies it.
