I wonder this too. It’s much harder to back into a cramped parking space than backing out into an open space. So they do it very slowly. Watching people do this is frustrating.
I'm not saying _I_ back into spaces. I generally drive into spots and reverse out of them. However, I admit that what I do is a tradeoff where I take on risk in order to have a mechanically easier time entering and exiting the space.
And then some parking spots require parallel parking which is best done via backing up anyway.
With cramped parking spaces, your real options are (a) backing into it or (b) driving forward into it. When you need to have a 90 degree turn, option (a) gives you more control over the eventual position of your car, and is frequently the only option.