I think it's because, when someone posts a silly question, it sends a message that the person is trying to have a little fun (or has inadvertently worded his question in a way that makes it appear to be a silly question . A lot of other people pick up on that invitation/opportunity to have a little fun and join in. Sometimes that may because people's "inner child" comes out (and a lot of people think it's really important that the inner child, or "inner wise-guy fourteen-year-old", get to come out regularly). Most people deal with grown-up realities and work most of their days, and either make having just a little fun a regular part of their day or else don't (in which case they may need the much needed break from all that seriousness).
Sometimes the silly question, itself, is the author's attempt to get away from hum-drum seriousness, or else it's his attempt to create some fun for others who see the question. I think, besides enjoying sharing some silliness with others in some small way, some people appreciate that the person who wrote the question was essentially making a friendly gesture to everyone else who reads the question. Then, too, when the person who posts the question was just in a negative mood to "contaminate" and otherwise serious set of questions; readers sometimes take it upon themselves to turn what was a negative motivation to post the question into something positive - fun.
Having said all that, boredom probably accounts for a lot of goofy questions and the number of answers they get. :)