The two are related, though. The decrease in tax rates was one of the main causes of the increase in startups, and if higher taxes made it harder to get rich from a startup, fewer people would do it.

Also, people in mid-century were richer than statistics indicate:

http://www.paulgraham.com/ladder.html

Which brings me to my favorite tax deduction ever. If you go back to the 50s, the most useful tax exemption available to the rich was that if they donated at least their annual income to charity then they didn't have to pay taxes. (This was called the flying nun exemption because the law was passed for a very rich nun who was sworn to poverty and hence donated her whole income to the church.)

How on earth was that useful? Well donations were valued at current worth, not what you paid. So invest large amounts in assets of increasing value (eg art), then a few years late donate the asset to a charity. Asset valuation could be made to grow by the simple expedient of donating large amounts of money to a charity you ran so that it could afford to buy a piece of artwork from you at inflated value. Then that set the valuation for the other pieces of art you chose to donate.

If you've ever wondered how art wound up costing so much, and how America's great public museums acquired so much of it, wonder no more.

Not so fast. (You even provided a counter-example.) It's no where near that clear-cut.

The Reagan tax cuts were accompanied by a huge reduction in deductions. (One of the biggies was the elimination of the deduction for personal interest.) The result is that how you made and spent your money became less important in how much you got to keep.

We are seeing more "no taxes for {specific company}" shennanigans now, but they're a different class of chicanery as they're basically payoffs to specific people.