I have no evidence, but I believe couriers like Evri are paid so many pence per item. So if they have an opportunity to deliver, rather than take it back and have to deliver a second time at their expense, they will take it. If this means leaving it on a doorstep in the wet, then that's what they will do. I don't support it, but if that's their business model, I can entirely see why the self-employed, gig economy workers would take this course of action. The only way to get them to do better is to take our business elsewhere, but we all like a bargain, so we, understandably, take the cheapest option presented to us.
EDIT: I should also add that the Hermes / Evri service in our area has always been excellent.
EDIT: I should also add that the Hermes / Evri service in our area has always been excellent.