Because spectrum is auctioned to the highest bidder by the FCC. This isn’t a secret.

Any given frequency band needs to have a clear owner so that it can be used effectively. Otherwise, you'd have a bunch of uncoordinated users transmitting over each other, and it would suck for everyone. Why is there three mega corporations rather than lots of smaller companies? Probably a combination of the large capital expense of building out infrastructure, and politics? Also, it's tricky to geographically partition radio spectrum without wasting it, i.e. give two different companies permission to use the same frequency in neighboring areas.

Radio waves are a magical gift to humanity, and there's a clear benefit to society managing the spectrum reasonably. I'm not claiming that that's currently the case or not, just that GSM is an old technology that is relatively wasteful of the radio spectrum it uses, and the same radio spectrum can be used much more effectively with more modern technology.