To become a billionaire, one must be able to amass 10 thousand times more wealth than the median household in the US. To reach the lofty heights of Bill Gates et al. one must amass ~ 1 million times more wealth than the median US household.
The reason why some believe that to be a billionaire is exploitative is that they believe this 4-6 order of magnitude disparity in wealth is inherently exploitative. You could always imagine a society where the warehouse workers were paid double ( or even quadruple ) for the effort they perform and a 100 billionaire would simply be a 100 millionaire.
Regardless of where you draw the line for the discrepancy to be considered exploitative. The line exists somewhere for everyone. Tsar Nicholas had the equivalent of ~1.3 trillion in today's dollars in wealth prior to the revolution, and I can't imagine the US supporting anyone with amassing more than 10-20 trillion dollars in wealth.
Carnegie et. al. each managed to earn up to ~300 billion in today's dollars prior to the anti-trust act.