Because they are a useless bunch of lazy dishonest incompetents.
I have been in the house all day, check my parcel status online and it says no-one at the delivery point, card left.
Not only is there no card here I am absolutely 100% positive that no attempt was made to deliver my goods, this means I will have to wait for monday which is no good to me and as a result I have had to go out and purchase some of the items from elsewhere as they are urgently required.
Love Scan, hate your courier service.