Reasons for Data Cap??

​I just wanted to understand why is Xfinity putting a data cap at all? ​
​I am comparing to Fiber Internet like ATT Fiber, Vexus Fiber, Verizon FiOS,  none of these plans have a data cap for high speed. Is this some kind of silly law FCC passed on cable companies? I just dont feel it's fair we have to pay extra 30 dollars to remove a data cap. Why does Xfinity even care how much I download or want to charge for it. It's an antiquated practice and feels like we are getting ripped off.  Fiber offers 60 to 80 dollars for a 1 Gbps plan that is synchronous up  and down but cable is nearly twice that cost with that fee for 1 Gbps down and a pitiful 38-42 Mbps upload. ​