I think it has a lot to do with the genrational thinking. I am of the 35 YO age group so I am in the middle in a sort of way. I grew up watching movies. Some of those were zombie movies. Also with the video games like Resident Evil shoot 'em up zombie games became quite popular. They still sell well also.
For some it is a means to get their flabby buttocks off of the sofa, and to the range. For some it is just pure for spits and giggles. Though there are always those who go way too far overboard.
I have shot my share of zombie targets that were provided for free from the range. Though I am cheap so I realy like to take advantage of freebies.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.