Except if bandwidth were legitimately scarce ISPs would be trying to incentivize the use of bandwidth at low times and disincentivize its use when congested. Power companies can and do do this - the price per kwh can be much cheaper at night or in the morning than by day or evening when there is less peak load on the system.

Nowhere are IPSs offering "3mbps from 5-10 and 30mbps the rest of the day" plans. Because the cost to improve total network bandwidth is almost nothing, relative to the last mile costs. Every ISP could easily meet the full rated line bandwidth of every residential connection they have within their current margins without issue. They don't, because they have no competitive pressure to actually do anything than make sure the current service works. They have no competitors besides maybe cellular providers, who are already paying them in business contracts to attach towers to the backbone.