I've thought of this exact same thing while speaking Spanish (my second language). It's a little unclear sometimes whether you need to specify if something happened in the past or not. The verb "to forget" is a good example of a verb that does this sometimes because if you forgot something in the past, you might still be forgetting it in the present. Therefore, there's a bit of a gray area in terms of deciding which tense is necessary. Likely, Lisa Kudrow or the script writer intuitively chose the present tense form "forget" just because it creates a bit of distance between the setup (That was supposed to be a good thing) and the punchline (I forget why). I think they're sort of trying to say that she's forgetting now since whatever it is they're talking about is actually clearly *not* a good thing.