Out of all the major sports, basketball must be the most difficult to reach at a professional level. A mid-tier pitching prospect playing AA ball may luck their way into a big-league start after a team suffers injuries or decides to rest its starters. You can be an undrafted free agent in the NFL and still get the chance at one play on special teams. Hell you hear about accountants and Zamboni drivers having to suit up as emergency goalies for NHL teams. You get my point.
But purely from a number’s standpoint, basketball is by far the most difficult.