Because there are emails and Facebook posts going around, saying that if you set your phone to that date, something amazing will happen - like one where it stated that it activates some retro mode and makes the phone look like mobile phones in the 70s (which is not that far from the truth). Some even have pictures of the iPhone with the old rainbow Apple logo.
It's interesting to see how many people here immediately assume that people who do this are dumb. There are a lot of "easter eggs" on websites, in electronic devices or in computer games. People are used to this kind of thing. So they think it's probably something funny, and then they do it.
For example, go to Google and type "do a barrel roll". Funny result, isn't it? Ha. Ha. Did you do it? Yes? If so, then what if this simple innocent little thing had bricked your computer or your mobile device? Then you'd be just as "dumb" as those people who set the date on their iPhone to 1970.
Yes, you are absolutely stupid if you take anything posted on Facebook, or an social media, at face value without researching it first. Like all the idiots who think a picture of a dog with a piece of ham on his face was a tortured pet. Idiots!