This model is simply an additional revenue stream for ISPs.
The same reason as to why you don't use your entire paycheque at the beginning of each month when it drops.
Granted though, in terms of internet, this is not a detriment (at least not enough) to think you won't have internet by the end of the month. But that'd the thinking behind caps, that people would even out their usage through the month.
*The normal plan gets deprioritized after a threshold. The cheap plan is always low priority. The super duper plan is never deprioritized but video throughput is capped.
Video streaming on any of these carrier networks consumed by your phone or tablet is curated for mobile devices and doesn't impact their Internet connectivity at all. Very little of that traffic will ever egress the carrier's network. Ultimately the congestion is all oversubscription of the spectrum allocated to the tower you're connecting to.
In the end you're trying to find a fair algorithm for "sharing" and any one you pick is going to have some ways in which it's unfair.
This is already done for almost every other wireless carrier with QoS (QCI in this context). No need to cap the heavy users, just de-prioritize them if the spots are too congested. [1][2] (QCI parameters may vary by carrier and region equipment vendor.) I spend most of my month at QCI 9 but I live in a semi-rural area with a high capacity cell pointed at me, so deprioritization doesn't matter for me, I'm harming literally nobody using >1TB/mo. If my town threw some festival and I bogged down, I have the choice of either switching to a deprioritized line or driving to a non-congested cell.
If they really have to hard cap it suggests they are either: 1. assholes or 2. built a poorly designed system that congests at more than the transponder spots.
[1] https://www.reddit.com/r/NoContract/comments/tn4733/qci_leve...
Interconnects are getting better and better so fewer and fewer people run into it (and how much video bandwidth never even leaves the carriers local network with Netflix cache servers, etc).