Obviously in terms of pedestrian safety, you're more likely to hit a pedestrian in the open/drive area of the lot rather than in a spot. So backing in is indeed safer for pedestrians. But for just about every other reason, it is impractical.

Also, valets are more practiced and often are parking in areas where only they are placing and moving cars. So it's a more uniform, controlled environment. The cars are probably lined up better, making it easier to get in, and the other cars are probably not moving at the time since only the valets move them occasionally.