Contrary to what you evidently imagine, circumstantial evidence can be perfectly good evidence, even in legal contexts (and remember, please, that legal categories of evidence are directed towards specific ends; to overgeneralise them can be misleading and a source of bad arguments).
When tool marks, builders’s graffiti (including cubit marks), dimensions in whole numbers of royal cubits, the support infrastructure for a large workforce, (many) inscriptions in the surrounding tombs specifying offices in the pyramid complexes of Khufu, Khafre and Menkaure, the special case of the Debehen inscription, which mentions a visit by Menkaure to inspect work on his (named) pyramid, radiocarbon results . . . when all of these are deemed not to be evidence, the only thing evidenced is the deemer’s evasion of the evidence.
If you want to be an agnostic on the question, fine. Just be a consistent one.