I think the postie often can't be arsed with dealing with these sort of things. If you aren't in and there is money owing on the letter, he'll have to take it back to the sorting office, and leave a card with you. I suspect that (i) a lot of the time they can't be arsed with filling in and leaving a card (assuming they haven't run out) and (ii) they don't want to carry stuff back.
I had a small parcel of TFL cycle maps left on my doorstep this week, I had ordered above some magic limit, which apparently means I have to sign for the parcel. I didn't sign anything. I've occasionally had parcels through the door with special delivery and signed for stickers, when I clearly haven't signed anything. It suits me, since I don't have to go to the sorting office to pick it up, and I guess it's only a problem for the sender if I (the addressee) complain that I didn't receive it.