Virtual particles are not are ordinary particles that pop in and out of existence. Virtual particles are a particular way to model quantum interactions. Some argue virtual particles are "real" in an ontological way, and pop science tends to lean toward that view, but they're certainly not real the way regular particles are real.
It's required that quantum interactions obey conservation laws, and any non-conserving "virtual configuration" must be short-lived, only existing to the extent it might affect interactions with "real", physically allowed configurations. There's no quantum interaction that lets a proton turn into something else while obeying conservation laws. In particular, normal interactions can't change the number of quarks in a given configuration, and the proton is the lowest energy configuration of 3 quarks. So it can't "tunnel" via a virtual configuration into some other real configuration.
Grand unified theories usually introduce additional mechanisms that can turn quarks into leptons, so proton decay is a test of those theories.
It's required that quantum interactions obey conservation laws, and any non-conserving "virtual configuration" must be short-lived, only existing to the extent it might affect interactions with "real", physically allowed configurations. There's no quantum interaction that lets a proton turn into something else while obeying conservation laws. In particular, normal interactions can't change the number of quarks in a given configuration, and the proton is the lowest energy configuration of 3 quarks. So it can't "tunnel" via a virtual configuration into some other real configuration.
Grand unified theories usually introduce additional mechanisms that can turn quarks into leptons, so proton decay is a test of those theories.