I think Macarther hit it head on. They were not a band driven by one or two people, but they all were firing on all pistons at once.
I saw them live. Once in 69 and again in 70. They were unbelievable.
Also, there is a musical gestalt that sometimes forms that is much more than just the sum of the parts. Zep and the Beatles, though radically different bands, are oftentimes compared. The Beatles were all average to average plus on their instruments. But together, they formed a cohesive unit that was difficult to explain.
During Zeps reign, their were other bands that could play as well as them, but they just didn't have that mysterious vibe that only Zep had.
I play guitar, and am, at best, average. I have known guitarist that were technically great,and could blow you away just noodling around. But, put them in a band, and they just can't find the pocket, that sweet place that you just have to feel.
At the same time, I know average players, who just after a few minutes can find a place in a band, and just sound good, and make the band better.
The Beatles and Zep both had that "play together/find the pocket" magic.