Also people get their GPAs from different classes, and you could theoretically not ever have taken a class taht would "test" your reasoning ability or whatever it is they test on the LSAT. It's also, like everyone else has mentioned, a standardized test that everyone takes on an equal playing field, regardless of difficulty of college, major, etc.
A really low GPA though could mean you're not a hard worker, which is why it's looked upon negatively, whereas a really high GPA could mean you're somewhat of a slacker that took easy classes, profs, easy major, etc.