It's a complete mystery, but there are a number of financially successful actors that are just not good at their craft. Maybe they provide such eye candy that we overlook their inability to act. Or maybe they are great at ridiculous potty humor that provides their one trick at being popular to watch. Or maybe they are just riding on past sucessful performances and taking any acting job that they get and are just phoning in their performances.
http://www.ranker.com/list/worst-prolific-film-actor/lanayoshii,
Adam Sandler
Arnold Schwarzenegger
Chris Hemsworth
Hayden Christensen
Jean-Claude Van Damme
Nicolas Cage
Dwayne Johnson
Tyler Perry
Vin Diesel
Will Ferrell