Fedex does not create their own electricity or fuel either, yet without those they have no going concern. Yet they choose to acquire those services on the open market because it is cheaper, even though they do make themselves more dependent on the supplier. In the end, full self-sufficiency is just not viable for large companies in a highly specialized economy.

In this case, if a cloud provider gets too monopolistic then the BATNA for fedex is to re-hire (or train) enough sysadmins to run their own systems again. That puts an upper limit to how much a cloud provider can charge for their services, in addition the competition between Azure/AWS/GCP will also keep prices somewhat down. The cloud providers know this and will price accordingly.