But it's a difficult thing to carry off. Several companies, I believe, have tried and all have failed. (I can't remember the names of any of those companies right now; I don't know if that's 'cause I'm tired, or if that's 'cause I'm actually remembering a conversation about a business that someone wanted to start that never happened, or if I'm just making this up wholecloth. None the less, it seems like a direction from which you could attack the market using unlicensed spectrum. You would have many obstacles, but it seems possible, at least in high-density urban areas.)
Landowners have a similar incentive. And, where it makes sense to do so, we see buildings that are many stories high, effectively using the same land many times over for the same purpose.
But in spectrum, we have spectrum lying around unused, while emergency services can't even get usable spectrum to save your butt in an emergency. Its well known that cops and firefighters couldn't even be alerted to leave one of the towers.
And this problem has still not been solved. But they can buy land with no problem.