Yeah, I never understood it either.
I have no problem with them doing it, I just never got the attraction.
I have shot most of the common sub machineguns and I didn't like the real thing: I personally wouldn't want to copy one.
A buddy of mine built a semi-auto, Browning .30 belt fed "machine gun" clone out of a parts kit. I praised him for the workmanship and I thought it was a cool project, but in the end he had a really heavy/bulky/totally non-ergonomic .308 rifle. If you wanted to fire it, and actually hit something with it; you had to use a tripod to support it.
But, it was kind of cool as a conversation piece.
I was in a gun store's indoor shooting range, YEARS ago and some guy had a whole pile of various sub machinegun clones. He rented the store's chronograph and was compiling all this velocity data. He ended up shooting the chronograph. Undaunted, he went back in the store, paid for the one he shot, rented another one and ended up shooting that one too !!!!!!
All the while I am thinking, WTH is this guy doing ? It isn't like you are going to get some wild/interesting/exotic velocity numbers: they all had a 16" barrel. Shoot a couple rounds out of one of them, and you are very close to what the other ones are going to shoot. But, I guess he needed some kind of stuff to fill in the pages of the book he was writing.
But, to each their own. I have said many times, on this board, that not everybody likes the same things as everybody else and that is OK with me.