Brand/name recognition + marketing.

It's part of the blockbuster model, which does everything it can to reduce risk. Before the 70s, studios would go bust when an expensive movie flopped. Studios became very risk averse, especially for the expensive stuff. So they make a sequel to a movie that's done well, or a plot similar to that of a movie that's previously done well, based on an intellectual property that sold well in another medium(comic, book, tv-show, ...), in a genre that's previously done well with audiences, starring actors people previously liked, preferably very attractive actors so that audiences like looking at them, pushed by a saturation marketing campaign that gets as many people to watch it on the opening weekend as possible, so that if it sucks they can't tell their friends not to go and see it. It's like McDonalds. It's not the best meal you'll ever eat, but you know what you're getting, so you won't have wasted two hours or your life, or shit yourself after eating it.

Also, video killed the radio star. It's rare to be incredibly beautiful. It's rare to be incredibly talented. It's incredibly rare to be both. If you have to pick one, pick the incredibly beautiful actor, who looks good on posters and in promotional material. Acting isn't that hard. Even a pretty moron can be a passable actor.

Tom Cruise has employees rewrite movies he'll be in to make his part more, and more in his style.

He has more acting range and ability than so many other actors