Well, why did they influence so many musicians if they were the musical messiahs many seem to think they were. Why have so many artists, from Frank Sinatra to the Grateful Dead, covered their material? Why have countless musicians named them as an influence? Rock & roll was a dying bree, corrupted by the popmeisters of the Brill Building and Tin Pan Alley by 1964, with schlock movie soundtracks by Elvis and teenybopper sludge being the norm, until the Beatles appeared on the Ed Sullivan show in '64. That was followed by a boom of garage bands and musical invention we haven't really seen since.
I don't think either Frank or the Dead covered EVERY Beatle song, but they did a select few.....