The network concentration
Every IAP (Internet Access Provider) concentrates tens of millions users.
Users, even if paying their subscription, become an audience that IAPs try to monetize to SP (Service Providers), and especially to those using lots of bandwith (ex. video streaming)
The servers concentration
The maths are simple: billions of users, a few thousands Data Centers. On average .
IAP to Data Center connectivity
To allow an acceptable quality of service to users, a very strong connectivity is needed between IAP and Data Centers. The cost of this interconnectivity is used by IAP as a pretext to ask SP (hosted in Data Centers) to pay.