Because communication requires both a talker and a listener and it takes time and energy for the listener to process information.  So it may be cheap to talk but it is costly to listen.

But then the cost of listening implies that there is an opportunity cost to everything you say.  Because you can only say so much and still be listened to. They won’t drink from a firehose.

When you want to be listened to you have an incentive to ration what you say, and therefore the mere fact that you chose to say something conveys information about how valuable it was to you to have it heard.  There is no babbling because babbling isn’t worth it.

I also believe that this is a key friction determining the architecture of social networks.  Who talks and who listens to whom?  The efficient structure economizes on the cost of listening.  It is efficient to have a small number of people who specialize in listening to many sources then selectively “curating” and rebroadcasting specialized content. End-listeners are spared the cost of filtering.  The economic question is whether the private and social incentives are aligned for someone who must ration his output in order to attract listeners.