Those companies run all their boring, essential software (accounting, ERPs, etc.) on VMWare, Oracle, Azure, etc. Since software isn't their core business, they pay an external company to keep their software running. Like it or not, cloud providers are for software companies, and most companies have no interest in becoming a software company. They might have some small groups that do analytics or software in-house, and those groups might use cloud providers, but all the essential software will be run by a company you can call when something breaks, or if you're big enough embeds some staff in your office.
It's just good sense to outsource non-core functions. Software companies outsource hosting and datacenter stuff to AWS, etc. Why should enterprises deal with Linux directly?