The WAFB piano riff

Why was this riff, or some similar sounding variation, everywhere in the late '70s and early '80s, even on music that isn't really otherwise yachty? Did it prefigure 'What a Fool Believes' and Michael McDonald got it from somewhere else? Were people consciously trying to copy the Doobie Brothers or was it just so prevalent that became the sound of that era and they forgot where it came from at some point?