1) The printing process is pretty difficult. There are multiple things you have to get right
2) There are people making fakes, most of them not good enough to pass as real cards.
3) The main point of having the real cards is to play in tournaments, where this stuff is enforced. If you are just playing with friends or trying out decks before settling on making one, it is common to make proxies of the cards you want.
4) Well made fake cards, like fake money will do Bad Things (tm) to the internal economy of the game.
There have been a lot of attempts, including a recent wave from China. However, there are unique features about how the cards are printed that make it difficult to reproduce: cards bend in a certain way, use a proprietary font, have a strip embedded between the layers, etc. High quality fakes can fool some people, but not usually those who can afford to fork over hundreds or thousands for the really valuable cards.
(not to mention if you use fakes in sanctioned Magic and are caught, big penalties can ensue)
I dont quite get it. You can print and laminate whatever you want at costco/home depot, why arent players making their own 'reproductions'?