Because it's not very efficient.
Actually, what it is is a policy that requires well trained sellers, familiarity with the businesses of advertisers, and frequent follow ups with clients once they "try" your station.
The system is very efficient, because clients renew in much higher percentages and that makes sellers more efficient.
I had a case years ago of a major consumer brands manufacturer which did not use radio. In the factory lobby was a sign that said, roughly, "if your business is radio advertising, we're not in".
I did some research with friends of the owner and found out that they had been on about 15 different local stations, and got no follow-up service, no real help with creative and no sales increases. Since the ad "buyer" was the owner, he expected to be called on by the manager or owner of a radio station or group, but he was getting the novice sellers.
I went to a hardware store and managed to buy one of those miniature doors that lock companies give retailers to show how their product looks... about 28" high, on a frame so it would open and close. We "fixed" the door to always be open, and then I took it to a sign painter, and had it inscribed with "If your business is selling more, our door is always open". My card was attached.
I delivered it to the lobby of the manufacturer. By the time I got back to the station I had a "call me" message. By the end of the day, I had a contract. I took our production guy and our best known announcer, and we worked out a campaign. We even got the host of one of my station's Quechua shows to do ads in that language for our "Before the Sun Rises" show. They advertised day in and day out every year until I moved on to another market.
As David Ogilvy said, "it's not creative unless it sells". And it does not sell unless you understand the client, their market and things like price points and the like. Direct accounts don't have an agency to do that for them, so a station that wants renewals has to look at how they can do creative that makes listeners want what the client offers.
That was the most efficient sale I may ever have made.