Why do Universities and Colleges still use letter grades?

[SBR]

I've never had letter grade. Since about grade 6 I have always been give a % grade. Why do most post secondary schools use letter grades, it essentially rounds peoples marks together. If person A gets an 84 and person B gets an 81 why the hell should they both end up with a A-. It can also screw with averages, if I get a 89 and and two 84s my average should be 85.6 (or an A) however if I were using letter grades my average would be an A- (which works out to an 83). Seems really [censored] stupid to me.