[apocolyps6]

why aren't people printing their own money?

a more serious answer:

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.