Hey Graham. Do you pay Comcast/AT&T to connect to their ISPs? If so how much? What is the benefit of setting up your own ISP if you need to connect to one of these giants anyways.
Things to note - you're not buying a regular residential or even a business line, you're (usually) buying a dedicated Internet Access line and dealing with an entirely different group for sales/support. It does seem kind of strange at first but I think the reality is that the raw bandwidth isn't one of the main costs for those companies, it's the support/last mile build&maintenance etc that's expensive to them - so if you're willing to give them some money and take that part off their hands they don't seem to mind.
Also - there are plenty of other companies selling fiber that don't also sell residential Internet products, so you're not always buying from them.