It has been covered in the NFA section 100 times before. It's because they said so.
If you want a nifty little .45 carbine (SBR Thompson) you fills out your forms, pays for your stamp and you wait like the rest of us.
If you want an awkward, heavy, very high capacity .45 pistol (Thompson pistol) buy that and quit your crying.
Seriously, shoot a SBR and shoot the same gun without a stock and you will know the SBR is the better gun. In the 1930s there was a problem with people shortening the barrel (but not removing the stock) of weapons so they could rob banks and such. That is why we have that law. Because they said so.
"No, it's not a silencer. This little doodad is my own invention. I call it a loudener." --Sledge Hammer