Most of these singers and groups quit recording 35 or so years ago.
And 35 years ago, the Billboard chart was based on wholesale shipments. The bigger stars, who generally did have loads of big hits, created an expectation of sales by retailers, who stocked up. Many of those songs were not hits, and the stores returned the unsold records for credit.
In more recent times, the chart has included sales of all kinds as well as airplay, eliminating the "hype" of new songs.
Michael Hagerty wrote a marvelous explanation in the LA forum and it is at http://www.radiodiscussions.com/sho...October-2014&p=6010696&viewfull=1#post6010696
The charts from the era you refer to are subject to all manner of distortion.