When most sports unions started professional players were not well paid. In the 1950s many of the best baseball players didn't play for professional teams because a local hardware store in some small town offered them better wages if they would work during the day and play on the town team on weekends!

>> I know that NBA players have a union, and top players definitely get rewarded well.

relative to what? the top players have always gotten big paydays, and the economics suggest without a union they would get paid even more. They are effectively subsidizing mid and lower tier players.