Consider the following:
1) you have to build out a certain type of network to provide a good experience, and this includes a minimum bandwidth allotment that is quite high.
2) because "normal" bandwidth usage is extremely bursty there is no reason (almost) to ever upgrade the bandwidth to the edge nodes.
3) bandwidth to edge nodes is constrained, even for incumbents. There's 2 cases, non-incumbents have to buy it from a monopoly, which is expensive. The other case is incumbents. They can get it cheap, until they hit a specific point (the bandwidth initial buildout that was done decades ago). When they hit that point it takes a massive investment to increase it.
4) the reason people go over bandwidth limits is peer to peer traffic. This unfortunately has two properties. Firstly, it's a consistent download, not bursty. Second it doesn't go out of their network (if it's a large network that is), meaning anyone that does this creates a constant traffic flow in TWO points of their network, and every component in between. This means that the bandwidth needed to support p2p is a factor more than the bandwidth your client actually reports to you, and exponentially more than needed to support browsing (even youtube).
5) If some ISP decides against this (they're small for example, in which case it all goes over cheap transit or even peering) that brings them extra costs, but not extra customers (because only the expensive customers like you, which can't be supported by the infrastructure, will switch over)
6) Most customers refuse to pay more. (e.g. switch to the business account)
7) Nobody is ever going to pay for a network that can support exponentially increasing bandwidth. It's not possible, for obvious reasons. When you're as big as comcast is, the investment for a 5% upgrade would require ridiculous amounts of capital, so they don't want to.
(replacing the current infrastructure with fiber MAY fix this, however the people who control the current infrastructure like Comcast have an interest in blocking that for obvious reasons. However there are problems with fiber too, the root of the problem is the constant light -> electrical -> light conversions that are necessary at lots and lots of points and make the capital investments quite high)