To answer your question...

Why don't people just start their own radio and news channels?

Well for radio, it's in part because you can't just start one up. there are no new licenses to be given out. The FCC controls broadcasting licenses. And they aren't giving out any new ones for full power commercial stations any time soon. probably not in our generation.

Secondly, it is ridiculously expensive to start up a radio station or TV channel, and those not getting their agendas out are typically those who don't have a lot of money to begin with! Then of course, they would represent competition to the existing channels, which are often partially owned by the cable networks. Therefore, even if you started a new cable news channel, there is no guarantee that the cable system would put it on.

You are asking the right questions. The media does indeed filter the information we need. And that is why we are fighting to reduce media concentration.