It's like the old Bell Phone, the big ugly black thing that was made by Northern Electric, that cost Bell about $50, that they rented to you for $2.50/month.  You could have the same phone for 20 years and it would cost you $600 in rentals, making a nice tidy $550 for the telephone company.  I think the only reason they quit doing it, was that there were too many people who had a phone that quit, and called for service, after all if it's Bell's line, Bell's equipment and Bells phone, then it's Bell's problem. 

Cable boxes are the same deal, you can bet that if you can buy the Box for $200, that cableco doesn't pay anything near that for it, probably $25 direct from the factory in China.  However they do need to pay to have a service technician replace it, when your service stops working.  The technician, the truck, the fuel, probably costs $200 for the service call.  It doesn't take very long for the service calls to eat up the profits on the box rental.  The problem for the cable companies is that there is no ISO standard that these boxes need to comply with.  So the cable company will pick a manufacturer and a couple of models, make sure their equipment works with that, and everyone else can just get stuffed.

If there was an ISO standard, or even a national standard that all boxes needed to comply with, then the TV manufacturers would probably include the proper circuits in the TV, you hook the cable up, call the cableco, give them the ID number off the back of the TV, they update your billing info and turn the service on.