> Economic efficiency would mean only a few vendors.

This is a massive lie monopolists tell you to justify anti-competitive mergers.

Increasing entity size has economies of scale (amortizing fixed costs over more units) and diseconomies of scale (bureaucratic overhead, long-distance transportation costs, etc.)

Economies of scale peter out with increasing entity size. Amortizing a $1000 fixed cost over 100 units instead of 10 saves $90/unit. Amortizing the same $1000 fixed cost over a trillion units instead of a million saves less than one tenth of one cent per unit.

Diseconomies of scale metastasize with increasing entity size. Entity size exceeds Dunbar number, language barriers and timezone asynchrony, corporate politics, independent jurisdictions imposing mutually-incompatible legal requirements, insufficient competition compromises incentives for efficiency as long-term incumbents succumb to Iron Law of Bureaucracy, etc.

By the time you're operating something at the scale of the entire planet, having the benefits of scale still exceed the costs of scale will happen approximately never.