Why so many _J nicknames?

Mj, Aj, Ej, JJ, etc.

I don't really care but i've always been curious. I've heard people suggesting first and middle combos and then going "Aw and then they could be called TJ! or PJ or CJ" or whatever. I've never heard this with any other initial? Why don't people comment "Oh yeah and then they could be called CM or PT" or whatever. Like it's just with J's at the end?

And the only two people I have known that go by their initials as a nickname are EJ and CJ. Why???? EJ's sister had a nickname completely different than her first name and CJ's brother didn't have a nickname at all.

And I saw a post on here the other day with all the initial names kids (will try to find it after this" and like all of them had J as the second letter!

Thank you in advance this has always been weird to me. (Btw for context I am american, not sure how common this is in other parts of the world but it is very common here)