Same as renting vs owning. Eventually you become the company laying the undersea cables and incremental increase in bandwidth utilisation effectively costs zero but you still have to pay to own and maintain all this infra.

The cost isn't just bandwidth, but also:

- infrastructure (memory, CPU, disks...): all of which must deal with the huge amounts of data they manage (500+ hours of video uploaded every minute).

- personnel cost (developers, SREs...) (almost) all of whom have substantial salaries that support the gigantic scale (1 billion+ hours of content viewed every day)