It’s obviously way more complex than that, though. The ISP isn’t going to make sure that all of their peering links have enough bandwidth to carry the full adve

It’s obviously way more complex than that, though. The ISP isn’t going to make sure that all of their peering links have enough bandwidth to carry the full advertised bandwidth of all of their customers connections simultaneously, in case everyone tries to download at full speed from the same website at once. On their end, the bandwidth is under provisioned. Data caps are a way of managing that that give customers higher peak speeds in exchange for limited data transfer.