This, I should think. The people waiting to get in to the lift won't know whether anyone will be getting out until the doors have opened fully and a second or two have elapsed.

It also takes a second or two to ascertain if there is indeed room.

And also, it is probably a normal, subconscious reaction for someone who is getting into an enclosed space when there are a whole lot of strangers already in that space to size up the situation quickly before jumping in...