The down and dirty reason is efficiency. They need a quick and easy
way of winnowing the field.
Of course, that's not the only reason. There are two more legitimate
ones. First, the LSAT is the one universal comparator that they've
got. How else can they compare a 3.8 in Basket-Weaving at Wombat
State with a 3.8 in Particle Physics at Awesome Ivy U.?
Second, the LSAT, for whatever reason, really is a SOMEWHAT reliable
predictor of first-year grades while in law school. In tandem with
the GPA, which is also individually a good predictor, they get a
combined VERY good predictor.
Of course, that doesn't explain why even two more points -
which LSAC themselves will tell the schools is statistically insignificant
-- can double your chances of admission. Them's just the breaks.
But -- you've got LSAT 180 to help you get those two more
points -- and then some! So what are you griping about?