Actors aren't allowed to do some stunts because of the dangers posed. Stunt people are educated and experienced with decades of experience behind them. Many actors need insurance to do their own stunts and not many insurance companies will run the risk of paying out hundreds of millions of dollars in lost earnings if an actor is injured on set.
Because they are wimps
To protect actors
Not everyone is Tom Freaking Cruise.
For really obvious reasons
When you pay a big name actor millions of dollars to complete a movie, you don't want them laid up in a hospital for weeks at a time from getting injured doing stunts. Stunt doubles are trained stunt people who KNOW how to do the tricks without getting hurt... Or if they do get hurt, at least they won't delay the completion of the film.
An actor is a valuable asset to a production company, and it would be foolish to put him or her in unnecessary danger. Stuntmen are trained to do dangerous stunts so the actors don't have to. And, if the role requires a skill the actor doesn't have and can't quickly learn, they would need a double to fill in for them in scenes requiring those skills.
Stunt doubles know how to safely do "stunts" -- falling off a cliff, fighting, etc.
The actors may or may not be able to do that.
If a stunt double does get hurt, there's probably another one to take their place.
If the actor gets hurt, it can delay the shooting and may even affect the rest of their career.
They use them so the real actors don’t get hurt
actors need stunt doubles because they are to important to the movies production to be hurt or risked the stunt double is there too dangerous scenes that may hurt the actors
Because they are clutzy.
So the actors won't get hurt.
Many simply can't do the stunts properly, and there is that pesky dollar value question: risk injury to the star (very costly to production and filming) or use someone else that an injury or accident will not force major delays. Also, many stunts are fairly tricky and require a lot of expertise to perform safely, and face it, if you aren't an expert at stunts, you shouldn't be doing them. Best person for the job is the stuntman. Most actors put their efforts into learning acting, not learning to do stunts.
Because some either aren't athletic enough or simply don't want to risk their own life for just a movie but there are people that are dedicated enough to do it for them
because they are too cowardly to take risks