are no cars or people impatiently rushing past. Plus, what Machine said above is correct, when going forward, the non-stearing trailing end cuts the corner making it much more difficult to get in.
When parking you are going from a large space to a small space so precision is needed. When leaving, you are going from a small space to a large space. Much more room for error.
For a truck, backing in is absolutely the correct way to park, and the bigger the truck, the more important it is.