I suggest that you familiarize yourself with Popek and Goldberg's virtualization requirements.
In particular, the VMX instructions are sensitive instructions: they can provide privileged access to devices and system resources. Therefore, access to the VMX instructions must be controlled. If those instructions did not trap, and there were no other special provisions made, there would be no way to control access to those instructions. In particular, in a trap-and-emulate architecture for virtualization, we need to ensure that those instructions trigger a trap, so that the VMM can emulate them properly when the guest executes one of them.
If they did not trap, and if no other special provisions were made, it could open up a security hole. It could also cause a loss of fidelity for virtualized guests.
You might also be interested in learning about nested virtualized (aka recursive virtualization).