Why are these companies contacting me so frequently?
Do they know something I don’t?

They contact you because they have found that landowners who are contacted repetitively will tend to wear out and start to believe their rhetoric (or in some cases, outright misrepresentations). Furthermore, most of the third party agents receive additional compensation for getting a deal done. In some cases, they receive no salary or compensation unless they sell. It is strictly good salesmanship, albeit annoying. For the tower companies, they prefer to get to you before someone else does because the price for the lease goes up. They also hope that by pressuring you into a decision, they can force you to avoid researching the situation (say, by hiring a reputable consultant like Steel in the Air, Inc.) and realizing the true value of your lease agreement. Lastly, the lease buyout industry is very incestuous in that many of the agents have moved from one company to another and have taken contact information for landowners with them. Each company reaches out to as many owners as they can.