Cloud infrastructure requires a massive, massive, massive amount of up-front investment. And below a certain % of the market, it becomes impossible to pay back that investment or even keep it going.
The industry is closer to large airplane manufacturers (e.g. only Airbus and Boeing), search engines (Google and a little bit of Bing), and similar.
Shoes, jam, and hosting services all require minimal investment. (Cars require a lot of investment too, but not at the level of building a cloud. Which is why there are more than 4 brands of cars, but not 4,000 like shoes or hosting services.)
EDIT: In response to comments below, I'm not talking about physical infrastructure here, which does scale rather than being largely up-front. I'm talking about the absolutely massive software investment in tooling and reliability. Not only are you building 1,000's of API endpoints across dozens of services, but you need to also guarantee 99.99% uptime and virtually zero data loss when required. And sure, a company like Facebook is the type of company that can engineer that stuff -- but it still costs $$$$$$$$$$$$ to build.
There are dozens of providers outside of the top 3, like DigitalOcean and Packet. FB already has a massive investment for their own infrastructure so the marginal cost is greatly reduced.
The industry is closer to large airplane manufacturers (e.g. only Airbus and Boeing), search engines (Google and a little bit of Bing), and similar.
Shoes, jam, and hosting services all require minimal investment. (Cars require a lot of investment too, but not at the level of building a cloud. Which is why there are more than 4 brands of cars, but not 4,000 like shoes or hosting services.)
EDIT: In response to comments below, I'm not talking about physical infrastructure here, which does scale rather than being largely up-front. I'm talking about the absolutely massive software investment in tooling and reliability. Not only are you building 1,000's of API endpoints across dozens of services, but you need to also guarantee 99.99% uptime and virtually zero data loss when required. And sure, a company like Facebook is the type of company that can engineer that stuff -- but it still costs $$$$$$$$$$$$ to build.