Arthur probably didn’t exist. Guinevere certainly didn’t. Even if they did, there’s no reason to assume any particular location would be the location of their tombs.

Plus, according to the legend, Arthur’s supposed to be alive and in the mystical island of Avalon, waiting until he is needed to return and become king again. These guys needed to look up their Geoffrey of Monmouth.

Basically, it would be like digging to find the tomb of Uncle Sam and his wife.