Because this limit isn’t about your hardware, but their software.
As appropriate a model this still is in the development VM scenario, you still need a valid license for each operating system copy you run.
Microsoft will sell you these individually; Apple apparently implicitly grants you up to three per Mac that you buy, and won’t let you pay for any more even if you want to.
In other words, what’s limited here is not really the hypervisor itself, but rather the “license granting component” that passes through the implicit permission to run macOS, but only up to some limit.
As appropriate a model this still is in the development VM scenario, you still need a valid license for each operating system copy you run.
Microsoft will sell you these individually; Apple apparently implicitly grants you up to three per Mac that you buy, and won’t let you pay for any more even if you want to.
In other words, what’s limited here is not really the hypervisor itself, but rather the “license granting component” that passes through the implicit permission to run macOS, but only up to some limit.