Pretty sure it's the license, specifically:

(iii) to install, use and run up to two (2) additional copies or instances of the Apple Software within virtual operating system environments on each Mac Computer you own or control that is already running the Apple Software, for purposes of: (a) software development; (b) testing during software development; (c) using macOS Server; or (d) personal, non-commercial use.

The grant set forth in Section 2B(iii) above does not permit you to use the virtualized copies or instances of the Apple Software in connection with service bureau, time-sharing, terminal sharing or other similar types of services.

Legal issues, I bet. AFAIK there's no way to run macos without Apple hardware and comply with the license. You could run a VPS provider on Mac Pros I guess, but I doubt it would be cheap.

Neither cheap nor virtual, macincloud.com

Not affiliated

There are, just not cheap compared to running something else.