> ISPs arrange peering accordingly

Two eyeball networks (consumer/business ISPs) are unlikely to have large PNIs with each other across wide geographical areas to cover sudden bursts of traffic between them. They will, however, have substantial capacity to content networks (not just CDNs, but AWS/Google etc) which is what they will have built out.

BitTorrent turns fairly predictable "North/South" traffic where capacity can be planned in advance and handed off "hot potato" as quickly as possible, into what is essentially "East/West" with no clear consistency which would cause massive amounts of congestion and/or unused capacity as they have to carry it potentially over long distances they have not been used to, with no guarantee that this large flow will exist in a few weeks time.

If BitTorrent knew network topology, it could act smarter -- CDNs accept BGP feeds from carriers and ISPs so that they can steer the traffic, this isn't practical for BitTorrent!