1. People are just getting stupider. Through the decline in education, our increased reliance on Frankenfood, and, among other things, the popularity of stupid things on television, it is possible that an increasing percentage of the population is becoming stupid, slack-jawed non-book-reading lazy slobs, lumbering around, doing and saying stupid shit. People are simply getting stupider and stupider.
2. People aren't getting stupider, but there are more cameras to catch more stupid things happening. With so many television channels, more people must bloviate more often about stupider subjects. There is also the cheap way to make television: stupidity as entertainment. Find a group of idiots, put them on television, and viola! a hit television show.
With cameras in phones, on dashboards, buildings and traffic poles, more stupid things are caught on camera, and with the Internet, those stupid things caught on camera are disseminated to more people, thus making it seem like humans are getting stupider.
3. The percentage of stupid people is not increasing, but there are simply more people, and while the percentage of stupid people isn't going up, the sum total of stupid people walking around doing stupid shit is rising. And since the chance of hearing about stupid shit is increased by increased surveillance, (see point 2. above) it SEEMS like people are getting exponentially stupider.
4. People are just getting stupider.