The general answer seems to be that religions (just like "total" political parties) provide value for money, in particular a social environment. Friends, baby sitters, group activities, help when you lose a job or someone dies. I think academics, in particular, tend to be such loners, and to be content with such social support as is provided by the government, that they radically underestimate how hungry people are for this sort of social interaction.
"5% give everything to the second player, nearly doubling our previous estimate of what percent of people are Jesus"
I wonder how much "windfall" or similar circumstances around the money change how one responds. In my recent history I've had two windfall gains, one an inheritance and one the money that was being handed out as the "everyone gets a check" part of covid relief. In both cases I was happy to give the money to family who needed it more than me.
I raise this because I don't think of myself as Jesus (not even by Scott's fairly undemanding tithing / rational altruism standards). I think the dispositive thing was really that this was windfall money; I don't think of "normal revenue" money in the same way. Would I consider money won while playing one of these games as windfall or as earned? I suspect it might be very fragile to the precise framing of the experiment...