One server is enough for many small businesses. And for large business (like X or Instagram) it is economically more profitable to own their servers. For example, in my country top companies like VK or Yandex own their datacenters and sell cloud services instead of paying for someone's else cloud.
Also if you have several servers you do not need to hire a full-time sysadmin.
> Managing a non-trivial hardware fleet requires people, and people cost money.
People in AWS also cost money and guess who is going to cover this cost?
Suppose that you are a midsize company or a b2b service, so you want to make sure that your service has minimum downtime.
This means that you need sysadmins in close proximity to your hardware to do hardware swapping/troubleshooting. Or you need to engineer your system to not have a SPOF (which is not easy). So you're looking at employing at least 2 engineers near your datacenter.