Why? Because, as I argued last week, they actually like being broadcasters.
And yet, for some reason, Adam Theirer at PFF got all huffy when I suggested that making licenses property would not change the fundamental human nature of people who go into the broadcasting business because, apparently, making something property changes your fundamental character.
Wouldn't it be nice to have a model of human behavior that was actually predictive, or even consistent? One of the privileges of property is to use it inefficiently. I do that with my house all the time, because I like it and its mine. The natural corollary is that grant of property rights will lead to inefficient use of resources. This may still get you the most efficient use overall (for example, as David Friedman argues, it maximizes the chance of getting efficient results by decentralizing decisionmaking) or there may not be a single "efficient" result (Coase be damnded), or we simply may have countervailing considerations (concern for fundamental human values regarded as inherent in private ownership). But the notion that private property will inherently produce the "most efficient" result (usually translated as the one I want from a policy perspective) is as ridiculous as assuming that regulatory processes always maximize efficiency (although I can make good arguments for the use of regulatory processes as well.
When dealing with human beings, reality is messy. Get comfortable with that or stop messing up public policy.