I find that there are still reasons for the society to continue with the door to door work:
1. It is a control mechanism. The monthly reporting creates a precise measurement of a persons compliance with the organization. This can be used for reward and punishment.
2. Busy work. They can't allow the members to stop and think. They must keep them occupied.
3. Money. Although donations are in the toilet, if it weren't for the monthly magazine order, no one would ever contribute anything. Any in some countries, they still charge. Considering the meager quality of the mags, any charge is primarily profit.
As for enjoyment in the service, I was baptized in 1978, and I have hated every minute of my approximately 3,000 hours in the service.
A couple of years ago, at the Saturday meeting for field service held by the CO, he asked the group why they go from door to door. His answer was that it was not to make disciples (because no one ever does any more). But, he listed about five reasons, including making God's name known, offering a sacrifice, and other lame rationalizations.