There is in fact a limit to how much bandwidth you have, it might seem like a fiber optic cable should have infinite bandwidth, but it doesn't. I never really thought about why there would be a limit but I just always remember thinking there must be something restricting the amount of bandwidth you have between two points, otherwise why wouldn't everyone have at least gigabit to their homes? Not only is there a technological limit (which was the first limit I learned about many years ago) due to having to process and handle the large amount of data coming in over the pipe, there's also a physical limit.
Basically, there's a limit to how short a pulse you can send down fiber optic cable, and the shorter you can make those pulses the more bandwidth you have. According to the video, the shortest pulse width is around 1 femtosecond which gives a maximum total bandwidth over fiber optic cable of 125TB/sec. While a very large number, it's certainly not infinite.
Plus, your posts seem to point out that you feel the costs to increase maximum theoretical bandwidth are trivial. It's not cheap supporting a hundred fiber connected gigabit devices, let alone the millions an ISP would need to support to give ever customer guaranteed gigabit bandwidth. It's not just the gigabit links that cost money, it's also the equipment required for the uplinks. You would need to be able to handle all of the traffic going back up to the tier 1 network, which is going to require a lot more than just a couple 1 gig GBICs. While I feel that plenty of ISPs are holding off upgrading their equipment since they can just continue charging their customers high fees for low speeds, I also don't think it would be feasible for any of them to start providing gigabit speed to everyone. Even 100 mbit is extremely cost prohibitive and not worth most ISPs resources. There are plenty that will provide guaranteed asynchronous bandwidth but they have a very small target market.