It's been 75 years since the last amateur, John Goodman, won the US Open. Only 5 men have ever done it. In 1913 Francis Ouimet won it. In '15 Jerome Travers did it. In '16 Chick Evans won. Then Bobby Jones won it in '23, '26, '29, & '30. Then, John Goodman won in 1933.
Since then there have been no amateurs win the US Open. Bobby Jones may have been an "amateur" but I think any of us would agree that his game was far from amateur. In today's day, he could make Tiger Woods money. And, if you are that good nowadays, you turn pro directly out of college.
So, let's add up the variable: the best players in the world turning pro earlier, and earlier in their careers, the prize money that is at stake in today's tournaments is amazing, with the advanced equipment of 21st century golf making players better and better, I could go on.
But, the question is will we ever see another amateur win the US Open?