I guess the sheer volume of shows that used to be produced, coupled with the lack of lucrative television employment, were big factors behind some of those numbers of shows.
I remember reading some years ago, I think on this board, where someone commented that something with Bernadette Peters was not as successful as it might have been had she not been 'over-exposed' on Broadway. this triggered me to look at BP's exact record on Broadway...in 61 years, she has been in 16 shows, her first for 2 weeks at age 11, and her second as a standby in a show that ran for 3 days, and her third on a play that ran for one performance. Leaving those out, and adjusting the years, that translates into 13 shows in 53 years.
It would be interesting to see how many actual performances those people gave on Broadway. A lot of those high numbers reflected a lot of short-runs, especially by today's standards, whereas -- continuing to use BP -- she actually played on Broadway for a cumulative total of 109 months, which I imagine could go against many of those people. Nothing particularly relevant, but another way of looking at someone's time on there Great White Way, number of shows, number of elapsed years, number of actual performances.