Simple: Visibility and speed. You look at a parking spot, and if it's empty, it's definitely empty. It's virtually guaranteed to stay that way as you back in, so you don't need to monitor what's in it. No cars, cyclists, pedestrians, emergency vehicles, et cetera, are going to enter the parking stall as you back in. That's not true of a street or lane when you back out into it. It's often difficult to even see traffic coming, as backup cameras don't have the wide-angle coverage, and there's always the possibility that you didn't see somebody.

As a result of both of those factors, with practice, backing in can be done in seconds, and pulling out is a breeze. Pulling in forward is a breeze, but for most people, backing out is a slower, more nerve-wracking maneuver. (At least that's my assumption from watching how long it takes.) On the other hand, the people who just YOLO it back out into traffic are psychopaths.