Because the intro is ten bars long and the links between verses are six and neither of those is divisible by four.
Because the chorus is traditional in a way that the verses are not and you don't expect the rock and roll, but they bring it!
Because the verses are four bars with two lines that rhyme, followed by five bars with two lines that don't (one long, one short).
Because the chorus is a bar of six and two bars of four, repeated three times, but it's not over. The traditional stuff sweeps it away, but even that is nuanced, with a plagal cadence.
Because they only give you the chorus once.
Because they give you SIX verses.
Because it's pretty much a three-chord song.
Because it's in the mixolydian mode.
Because that voice (and the harmonies on the chorus).
Because they sound like Can and Neu!