I figure a lot of it is the same reason people buy fast cars even if they never take it to the track: bragging rights. They want the best just for the sake of it, and anything that challenges that is an affront to the goal.
Perhaps I am not TOTALLY immune to that myself, I could have built my current machine for a fair bit less than I did, but I wanted a 950 Pro because reasons since I've never had a fast drive before (I love the load and boot times, but I do admit, the extra second or two a regular SSD would take isn't much in the grand scheme of things), though at the same time I wanted an M.2 drive for practical reasons, so why not get something modern than just a repackaged SATA.
I think the annoying bit is some people not being accepting of requirements aside from absolute performance. People seem to love hating on AMD, but they are still in business because it's easier to make a decent performing budget PC with them. That's why some of my previous cards were AMD, and I jumped on the AMD bandwagon when Eyefinity came out specifically because at the time Nvidia Surround required the purchase of two cards. Obviously for actual 3 screen gaming, the extra card was a boon, but for just rendering the desktop, it's useless.