I can't tell you why Christians do that.
I can tell you why I do it though:
1. I like different. I get bored with the same ole same ole stories all the time. (How many crime shows, doctor shows, "reality shows, "scandal" shows, and dysfunctional family comedies can Hollywood dump in the time slots? Boy! It gets boring. I can even teach you a plug-and-play formula on how to figure out whodunit from the beginning of just about any crime shows that works 95% of the time. That predictable.)
2. TV watching wise, or even book wise, what else is out there but worldly-ways? Sure, there is Christian stuff, but, until recently, something I said when I was young mostly still applies now. "Christian contemporary music is neither Christian nor contemporary. It's middle-class America being taught as Christian. And it's 20 years behind the times." I think the only change since then is now it's 40 years behind the times. But look at what we get stuck with as Christians. LAME! The vast majority of Christian movies and book get a big fat LAME! I've been looking for it for 40 years. It's only been in the last 2-3 years I've actually seen/read something that was genuinely Christian and good.
3. I am an original Treker. I grew up watching the original Star Trek on TV, and I was a kid back then. I could also spot the crap in it a mile away. Yeah? Really? Man "evolved" into peaceful in the course of 400 years? Then why were they fighting in just about every single show? Man doesn't use money anymore? Then what do you do if you don't want to go exploring in space? Be a farmer? What for? To feed other people? Well, there are replicators, so no one is going hungry. And can you really just give away your produce because you know someone else is building a machine to harvest your crop and will give that to you for free too? And Spock was emotionless? Than how did he have friends? Having friends is done through emotions! Making babies is done through emotions. So, I really have been able to not buy the Kool Aid even when I was a kid.
4. Buffy? Buffy is too funny to not watch. lol