They have a physical monopoly in this case with access to the wires. A startup can't just roll out their own network everywhere, even with unlimited capital, due to the legal rights needed to lay that cable. You also can't choose to not use them anymore as they have become an integral part of society. Many things you used to do in person can only be done on the internet now. At this point there is no functional difference between paying protection money to a gang or paying the ISPs. Your business will be trashed, your own personal livelihood can be destroyed due to lack of internet access, and the government will back up the isp if you fight it in any way.

That is why it's bad that all this does is make the rich richer.