1. Imagine if Amazon starts building datacenters for others as a service
2. People starts leasing or renting entire DCs built by Amazon, Google etc. Maintenance is done by these cloud providers.
Do we have these already?
Just as an FYI, those fancy datacentres you see on Amazon/Google/Microsoft marketing videos are really the exception rather than the rule.
You will find the majority of cloud servers in exactly the same third-party datacentres that everyone else uses.
Why ?
Because only the US and perhaps one or two other countries in the world has the spare land for a cloud operator to dump a massive datacentre campus on.
Most other countries don't. Or if they do, its either protected land (greenbelt etc.) or its uninhabitable (e.g. Australian Outback).
There's rather a lot of land in the world outside of the US that could have a datacenter built on it. There's also a lot of Australia that's uninhabited but not uninhabitable.
No doubt other things like local laws, tax rebates and whatnot also come into play as well.
You may seek to argue that there are a small number of third-party datacentre sites where the cloud operator is the sole tenant of the building. But this again is not the same thing as the cloud operator building their own. They get the option to up sticks and leave at the end of their contract. They also don't have any responsibility over facilities management etc.
At most sites, the cloud operator simply has whole or part of a floor (or floors in larger buildings), the rest of the building is occupied by third-party customers.
AWS, Azure, Google Cloud, etc are the alternative to dealing with dedicated physical infrastructure and financing. Things like AWS Outpost are there to bridge the gap for companies/workloads that dont fit existing public cloud offerings.