You're making ill-formed and smug assumptions about how door to door preaching groups organize their time and manpower.
It is entirely possible that I am smug - it has happened before.

I am not, however, ill-informed, and YOU are the one making assumptions about how much personal knowledge and insight I may have into how evangelism works. I, personally, am not for evangelical behavior, but one entire side of my rather large family was and is, and I grew up around them.
But, if it allows you to continue to feel like you accomplished...something, I'll leave you to your assumptions.
I'll repeat,
Repeating is not leaving me to my "assumptions". But based on your apparent understanding of the physical laws of the universe, I'm not surprised that you would have a different perspective on that, too.
they plan ahead for interactions like the ones you seem satisfied in engaging, and compensate according. Again, despite what you may think, you kept them away from no-one they wished to reach that day or any day you engaged them. You wasted your own time and the people who came to your door no doubt had a laugh at your expense once they left.
Okay, I'm going to try this again, in more detail. A team of two evangelicals comes to my door, and has an hour long conversation with me - let's say, from 2PM to 3PM. Those two cannot, therefore, be having a conversation with someone else during that same period of time. Unless you are suggesting that G_d somehow compensates them for that time by allowing them to live an hour longer at the end of their lives or some such, that hour is GONE for them. They may go on to talk to other people from 3PM - 6PM, and then go home for the day, but they were going to do that, anyway, and that in no way negates that I tied them up for an hour and kept them from talking to other people with whom they may have gained more traction in THAT hour. It also doesn't matter if 6 other members of their group talked to other people in that time, since they'd have been doing that anyway.
The only way I can NOT have wasted their time is if it is only the fact that they tried that counts, as far as they are concerned. In which case, we are back to my aforementioned Win-Win, since they got what they wanted out of it, and so did I, which was to delay them in talking to other, possibly more susceptible people.
Similarly, I believe you will find that I have wasted your time in reading this, thus keeping you from harassing other people with your lack of logic during that time. ;-P