Putting my parcel delivery head on...I was in the parcel delivery game for almost 40 years for a well known company based in Sheepbridge.
The reasons things happen are usually straightforward, they started using scanners as proof of delivery and lately want you to return items in single boxes mainly because it's all down to saving time but also trust.
When I started delivering parcels many moons ago I had a rural{ish} round so I could quite happily leave many parcels on the doorsteps, without gaining a signature, safe in the knowledge they would be there when someone got home, I think in one five year stint only ONE parcel on my route was classed as "not delivered" a.k.a. pinched and even that parcel, as it turned out, had been delivered by British Rail {yes they delivered as well then}
Then the rot set in certain people found it a money spinner to claim they had not received a parcel when they had, plus nicking parcels off doorsteps became more prevalent to the point it became a full time job for many a tea leaf. hence the introduction of scanners, this is why parcel deliverers go from house to house if someone is not in to try to leave with a neighbour and obtain a signature
{obviously Covid has curtailed this practice to some extent for a while}
Sending back parcels in individual boxes usually means that it saves the depot having to sort through a box with multiple items in checking what can be re used and what can't, plus people could claim they had packed more items in a parcel when in fact they hadn't making it very difficult for the company to prove otherwise. Those items that can't be re sold usually go straight to an auction site to be sold off en mass to the highest bidder.
I sent back a parcel a couple of weeks ago to Amazon. I sent back two pillows that when they arrived were shrunk wrapped and each one was about as big as 2 toilet rolls end to end. They arrived in a box that was about the same size as my kitchen fridge and the brown wrapping paper {obviously ripped off a large roll} was so long I actually measured the length of it and it was 75 feet long. When I took the box to the Post office to return it the box was so big she couldn't fit it through the P.O. counter door.