because apple has copyrights on it and that they keep schematics secrets... they never went out of the factory, may be even from the design room...
the only schematics I know of are the macintosh plus ( year 1986...) so very old.
by the way, what do you want to do with these schematics ? repair one ? explain the defect and I can help you in finding the fault. be aware that lot of circuits in it are made by apple, and not available.. believe me the best way to repair such computers is to have two or three of them not working, and make one working from the others.
I did this with an SE/30 some years ago...