But we're not talking about a monopoly here. For example for cloud hosting, it's competition between Amazon, Google, Microsoft, OVH, and perhaps a few more "big" players.

Like in many technology fields, you have high fixed costs that are distributed over all of your customers, and economies of scale. It is much cheaper for Google to add 10 000 new servers to their datacentres, compared to starting up a new hosting company, building small datacentres on five continents, and rewriting all the software that Google Cloud offers on top of servers.

So seems plausible that a highly concentrated industry with a few big companies each having >10% market share is more efficient (and can offer lower prices) than a market with thousands of small artisanal hosting companies with <0.1% market share.