In some cases yes you are buying an upstream connection from Comcast/AT&T/Centurylink/etc. I actually have a note about that specifically on the site, as it happens! (https://startyourownisp.com/posts/fiber-provider/#overview second paragraph)

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.