Because objectively there is a limit on bandwidth that grows slower than streaming business and it is unfairly distributed among users in general. You can think of 10 items or less as an analogy. If I try to use my LTE from 17:00 to ~19:30, my bandwidth is almost dead because everyone returned from work to watch their feed. The only way to return my fairness is to storm the network with millions of torrent packets or video read-aheads. If ISPs would sorry me and charge less (I get exactly the same bill, btw), then it is equivalent for them to simply charge heavy users more or throttle them down.

Edit: it is worth noting that ISPs hugely oversell their bandwidth.