Take a look at some answers to frequently asked questions about the Presidents Cup:
What's the big difference between the Presidents Cup and the Ryder Cup?
In the Ryder Cup, the United States plays against Europe. In the Presidents Cup, the U.S. plays against an "International" team, meaning all countries EXCEPT Europe. The Presidents Cup is held every two years, in non-Ryder Cup years, which 2019 happens to be.
So, the Americans have to play one of these cup events every year, while the foreign players get every other year off? Why don't they just combine the two events?