People spend so much time watching TV that it becomes their main source of infomation on everything.
It may seem that way but if you think about the influence of media in general then the impact of TV is not nearly the whole picture. You have to factor in things like advertising, billboards, the rise of the internet, and more important than all of those things is that people are still social creatures and the opinions of someone you know has significant weight. Particularly if you categorize that person as an opinion leader.
The Internet is quickly displacing the TV as a source of information. Thats complex because the Internet has fewer gatekeepers and many more sources of information than TV does. Its easy and relatively cheap for me as a user on the net to disseminate my message and I could do it in a very media rich sort of way with comparatively little knowledge versus me trying to get time on at TV station or run a TV station.
Also what country you're in also makes a big difference here.
EDIT: Oh and I forgot to add...its been generally thought that whenever a new media type (and ultimately a source of information) arrives on the market that it displaces the old sources of media. This also doesn't seem to be the case. TV never displaced radio which never displaced newspaper. All of which still exist although in modified forms...newspaper is still very important as an authoritative source of information, however, its often now available in online format. So I'd say that TV is not the only source.