The market sets the prices of the services at what people are willing to pay. If the cable companies could charge more for internet than they are now, don't you think they would? People are willing to pay a lot more for television service,[1] so if the product you're selling is TV+internet, you can charge enough to justify building a modern fiber/coax network. If it's just internet, you probably can't, at least not in many places.

[1] In my building, in downtown Baltimore, I can get 50/15 FiOS for $50/month, no cap but no TV service, or triple-play cable at 50/5, capped for $109 per month. Almost everyone subscribes to cable rather than FiOS.