On Alec Baldwin’s Shooting

We recently learned that Alec Baldwin is being charged with involuntary manslaughter for his accidental shooting on a movie set. I don’t know the details of the case, nor the intricacies of the law, but I have a question about movie props.

Why was an actual gun used on the set? And why were actual bullets used on the set? Why wasn’t it a fake gun: plastic, or metal without a working barrel? Why does it have to fire blanks? Why can’t everyone just pretend, and let someone add the bang and the muzzle flash in post-production?

Movies are filled with fakery. The light sabers in Star Wars weren’t real; the lighting effects and “wooj-wooj” noises were add afterwards. The phasers in Star Trek weren’t real either. Jar Jar Binks was 100% computer generated. So were a gazillion “props” from the Harry Potter movies. Even regular, non-SF non-magical movies have special effects. They’re easy.

Why are guns different?