Because arguing in absolutes is easier. Take Obi Wan Kenobi's words: "Only the Sith deal in absolutes!" Presumably, this is because Jedi put much more time into their thoughts and actions. It is easy to fall to the dark side and become a sith. Jedis have to be much more careful, and this carefulness is hard, as it were.
In terms of pure argument, your best bet would be a qualified essay. This basically means everything between two extremes. If you don't know how to properly qualify an essay, please learn for your own sake.
If you have trouble writing about extremes, then write a qualified argument. Taking the middle ground is acceptable (got me an 11 on ACT I think), but it takes more time. It also requires a bit more critical thought, because you pretty much have to define your middle ground. I pulled it off because I'm a fast writer. Again, it's easier to just take one side. It's slightly harder to qualify that one side. On some of these prompts though, a middle ground essay, pulled off, is what is necessary.