Why doesn't VNC Server start on boot on Windows / MacOS?

If you have Bitlocker (Windows) or FileVault (MacOS) enabled, then these prevent all applications, including RealVNC Server, from starting on boot.

Bitlocker and FileVault apply before the operating system loads, so no software, networking etc. can be started until the system is unlocked.

If you need to remotely reboot a device while it is unattended, you will need to ensure Bitlocker/FileVault are disabled before you reboot, then re-enabled afterwards.