I’m very curious, why did Apple put such a limitation?
Because their business model is to sell tightly integrated hardware and software as a package. The hardware sales fund the software development. They don't want people who haven't bought the hardware using the software.
My 2018 mac mini officially supports VMware ESXi to be installed directly on the hardware and virtualize any number of macOS machines
Funny enough I can even launch more than 2 macOS vms on my framework chromebook with qemu + KVM from the integrated Linux terminal.