Because - at least the way it's celebrated around here and by the people I know - it is such an innocent and fun holiday. (I know, I know there are pre-historic pagan ties and costumes in the stores that leave a lot to be desired.) But around here, it means dressing up in something fun (and I didn't see too many costumes that were gross or offensive or anything like that) and going to people's houses to get candy. The people in the houses are happy to give the candy - the children are more than happy to get it.
We respect people's rights not to be disturbed, and if their porch light is not on we don't stop. In our area people sometimes leave their candy out front in a bowl - that way if they are out trick-or-treating themselves they can still distribute their candy - and mostly I think people respect the honor system method of that.
The weather was beautiful, the kids had fun - and now have enough candy to keep them on a permanent sugar rush until Thanksgiving. We gave out all our candy (though the last of it was distributed on the honor-system, and I can't be 100% certain everyone was honorable).
What's not to like?