Nothing makes a joke stop being funny to kids faster than an adult trying too hard to get in on it! Some day when they're older they'll realize that was your goal and the cycle will continue.
It just gets stuck in your head and is something to laugh at. The kids who are into it are too young to remember the other catch phrases we got into. My four year old repeats it all the time, which keeps it stuck in my head.
I'm just waiting for Sesame Street to declare 6 and 7 the numbers of the day.
You not getting it is the main point of the saying: it's slang term that young folks (in-group) are using to differentiate themselves from the not-young folks (out-group).
It originated as a brag - a basketball player said it like "I am 6' 7", therefore I am a high quality male and women should reproduce with me". Then shorter people picked up on it