because ak's have a huge varience in dimensions from one type to the next. even amongst the best brands of ak there is a lot of discrepency in dimensions. fitting may or may not be required.
if you need to fit it only remove a very very very little at a time. hand files work fine.
just remember its easy to remove metal and damned hard to put it back.
when i hand fit parts i typical do it one file stroke ata time between attempts to fit.
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." L. Neil Smith
The problem with being stupid is you cannot simply decide to stop doing dumb things...