There's a lot of kids taking it, and it needs to be able to differentiate kids in a country with very wide ranging abilities. If you made it harder there'd probably be a whole load of people bunched up at the bottom.
Okay serious question, which has the larger negative societal impact - making it harder to differentiate between the top tenth of a percent of college applicants, or making it harder to differentiate between the bottom ten percent of college applicants?