VM execution requires computing power and disk space. Those are both naturally scarce, especially in the world's most redundant VM. The same VM instruction is run on more physical CPUs and its result is stored on more SSDs than any other VM setup in history. The overhead required for this kind of redundancy is insane, and it's why the Ethereum VM has such extremely limited throughput.
If the VM's cap were lifted and it allowed anybody to use it for free, the demand for VM resources would quickly exceed the clients' ability to host the VM. The Ethereum VM would pin even the beefiest host's CPU at 100% and require each network participant to maintain petabytes of disk space.
The way Ethereum works, every client needs to execute every calculation that every other client wants to run on the VM. Every request to run code on the VM is basically an amplification attack against the network, which is why execution is strictly metered and charged for. It's a ballooning On^2 resource problem that is very much limited by the natural scarcity and limitations of the silicon it runs on.
