> Having a flat price for internet sounds similar to having flat price for electricity or water - where your grandma (who turns off all the light switches whenever she leaves a room) would pay the same for electricity as a frat house mining bitcoin in the cupboard.
The difference is that to provide you with more electricity, the power company has to burn more natural gas, and pay more for that. When a data network is at less than 100% capacity, higher usage doesn't cost anything, the extra capacity is just lost. So to begin with, data caps that count traffic during off-peak hours are a pure scam; there is no reason for anyone to conserve bandwidth then.
Then you have the on-peak hours. Now, there are different ways to handle this. One is to set pricing that will deter usage, the other is to share the capacity. So you have a 1Gbps fiber connection but the uplink is oversubscribed and during peak hours you each get 800Mbps. People generally prefer this to paying more.
But some people want to have the 1Gbps per second all the time, so they should prefer metered billing, right? Still no. What you have instead is business service. If you want to be sure you have 1Gbps all the time, you pay a higher monthly fee and then get a higher class of service so that you do. Now the lower cost connection gets 1Gbps for 80% of the day and 500Mbps during peak hours -- which is a completely reasonable trade off for your grandma because she gets to save money and 500Mbps is still plenty enough. Whereas if you have some important need to always have the full speed, you can pay more and get it.
There is never any reason to meter it, and some pretty good reasons not to. It screws the people who actually need the guarantee if the metered pricing is ever insufficient to keep the network below 100% capacity. It also makes inefficient use of available capacity, because then you need to keep the network below 100% capacity, but that results in wasted capacity. And, of course, it creates a perverse incentive for the ISP when they can exempt their own services from the cap.