The PGA Tour determines who plays in a tournament through a system of category exemptions. Players in the top category get first crack at any tournament they want. Then players in the next category get an opportunity.
So, for example, the first category is for winners of the PGA or the US Open prior to 1970, or in the last five years (those who won before 1998 have a ten year exemption, so this runs out this year). Once those players have had their chance, the winners of the Players in the last five years are penciled in. Then winners of the Masters, Winners of the Tour Championshiop, winners of WGA events, winners of Tour events in the last five years. And so on.
What this system does preserve “jobs” for those who already are members of the tour. If you win an event, you don’t have to worry about getting into any Tour tournament you want for the next five years. If you pick your tournament chances well, that makes it more likely that you’ll win a second time and extend that exemption. Players who are high on the exemption list also have a much easier time of staying in the top 125 in earnings required to keep their card from year to year.
Conversely, it also makes it difficult for new players to break in. Even players who have sailed through Q School will find (often to their surprise) that they are often shut out of the system. That can make it somewhat difficult to get into the required 15 tournaments—and that much more difficult to stay in the Top 125 in earnings to keep that card.
That explains the oft-lamented lack of new, young talent. They’re shut out by the old-boy system.
Below is the complete and official list of exemptions: