Why is an Oracle Virtual Box required for Windows installation?

Last verified against genymotion.com:

No. On Windows, Genymotion Desktop uses VirtualBox as the default and recommended hypervisor, and QEMU is not the Windows recommendation.

Genymotion Desktop for Windows is designed to run with VirtualBox, while the macOS docs say QEMU is the preferred option there. The product also only provides Windows support for x86_64, and the download must come from Genymotion Desktop download.