I think that Vitruvius was aiming deeper than that of Boyle saying that beauty was delightful.

The Greek from the Sanskrit was Kallos, meaning handsome. Hellenized it became ομορφιά (omorfiá) derives from the word hour. Or young and mature.

I believe because of the Firmitas component of the equation, Vitruvius is trying to state that beauty is more lasting. The idea, that styles change, but the structure needs to always be firm.