Hmmmm... because they're actresses and actors, and not stunt people, cops, dancers, astronauts, Biblical figures, and so on?
I think I may have some bad news for you.
William Shatner didn't really go into space or command the USS Enterprise or kill alien dudes.
Judy Garland didn't really go to Oz or make friends with Munchkins or land a house on anyone's sister.
Clint Eastwood didn't really stroll around San Francisco with his .44 Smith & Wesson Model 29 blowing away bad guys.
Slim Pickens never rode the H-bomb to detonation.
Charlton Heston never went to the Holy Land to part the Red Sea, never traveled into the future to kill any damned dirty apes, and never offed any zombies after a terrible plague took most of humanity. He never thought that Soylent Green was people, either.
They were acting. Hired for their ability to deliver lines or look pretty/handsome, or because they made nice with the producer or because they owned the property.
HTH
“Galaxy Quest” was an amusing movie in which aliens come to earth looking for a great commander who can rescue their civilization. They have seen the TV show “Galaxy Quest” and are convinced that the William Shatner-like lead actor can command their starship and do the job.