Because before opening a door we have to check that the person asking has the right to enter. It is what stops a stranger getting your home opened just by saying they live there.
It can seem a nuisance when you are standing in the street in your pyjamas and all you want is to get in. But think of it the other way round: if anyone could ask for a door to be opened without proving anything, they could have yours opened too.
How it is shown depends on the case, and we explain it when you call. There is normally a way of doing it even if you have nothing on you.
If someone cannot or will not show their right to enter, we do not open, at night too and however urgent it seems.
We explain it in door opening.