Do people in commercials only get to do commercials?

No - many actors got their start in commercials. Bryan Cranston comes to mind. Wesley Snipes, Ben Affleck, Morgan Freeman, John Travolta, and Lindsay Lohan also started in commercials (8 Big-Name Hollywood Stars Who Started Off in Commercials). read more

Commercials are highly desirable jobs because an actor is not only paid to make the commercial but usually is paid every time the commercial plays on every channel throughout the world. In a popular commercial an actor can acquire not only a lot of money for no additional work but because they are seen by millions of people a day they sometimes become famous at least for the time the commercial runs. read more