Atlassian doesn't sell Cloud licenses for instances beyond 2,000 users. At that point, you need to dial up your own "on-premise" Data Center version yourself on AWS in order to scale further - not to mention federated use cases, which Atlassian themselves employ, e.g. a public-facing instance for tracking suggestions and public bug reports, and an internal instance for development itself.

Atlassian Cloud hosting is for small players.

Some industries (financial for example) have to meet regulatory/audit requirements that don't allow for cloud hosting.

Aerospace and defence too.

Sure all those smart people who were hosed when AWS East was having issues and their whole development and deployment pipelines got hosed because of all the SaaS services that went down.