We train so many physicists and their are so few jobs worldwide that a physicist has to be willing to travel all over the world to get jobs. When the average physicist works in several countries before he/she lands a position, they forge collaborations all over the world.
Just in graduate school, I worked in three different countries, and thats not that uncommon. My collaborators were all over the world.
Now, if you are the rare physicist who works on something with industry applications, you may have to keep things closer to your chest. But odds are, your collaborators will still be international, just not as open.