Because witchcraft, sorcery, and magic is cool. Think about it--arcane powers to manipulate the physical world, fight enemies, and simply mess around with people. That appeals to a lot of people, especially since magic is unlimited--you've got imagination, you've got a unique and engrossing magic system.
..cause they're bored with their real lives.... all about excitement and they'll do anything to get it... i don't read/watch anything with that stuff in it..
Locke Cole above said it right. Its just ricking cool. I mean in the Wheel of Time threes this battle for example where literally this guys appear from thin air and start blowing peoples' heads into bits with their magic. (that's a more gory example), but where else do you get that?! its class. fireballs emanating from the character's hands, stuff like that really excite me. I can't count how many of them I've read and yet starting a new world, with new characters and new magic systems is just as exciting and still different.
I like it because it's not reality. And the characters in those books have their own pr obs and it gives me a chance to escape mine in my own life. Some Pol may even believe in some of it, like I believe in Dragons.