Note the rather primitive recording technology of the day required the vocals be done simultaneously, while today they would probably be overdubbed one at a time.
Believe it or not, most of us still do it old-school in order to catch the feel of the moment. The lead vox gets laid down and then we gather around an omni-directional mic and sing it choir-style. It's just a better sound and feel even though the ProTools set-up does allow for singing the parts solo and even doubling them up. It's just too cold and insincere for my taste.
Don't even get me started on those pitch-correcting programs.