Because you can still get a 400 without doing anything.
So you get a 17% for doing nothing. Big deal?
Percentages would provide a false representation of your score. You say getting a 400 is 17%, but it really isn't. 17% would imply that on the test, if it was 100 reading and 100 math questions, one would get 34 correct, which in the case of the SAT, is incorrect. For the raw score, you get 1 point for a correct answer, 0 points for a blank, and -0.25 for an erroneous answer. Using percentages wouldn't be logical in this case, imo.