I think tees are "flat" because that does not discriminate against any segment of players. Everybody starts from the same point, with a level stance which does not favour any players. If tees were canted markedly to either direction, the slope would dictate play more than is desireable. It is better to let the ground contours of the fairway or hazards ask players to work the ball a certain direction.
Tees are not perfectly flat, a slope of approximately 1% should be incorporated to facilitate surface drainage.
TK