What's with all the reboots and replacing Actors ?

Reboots , Reboots ...every time I turn around there is a new Movie reboot with new actor or actors. There was a new batman actor for batman , there was a new cast for The Incredible Hulk" movie , there was a new actor for War Machine in Iron Man 2 , there is a new Super Man actor coming , when the new Super Man franchise was just getting started with "Super Man Returns" ...now there is a New Hulk actor and New Spider Man coming as well as New cast for X-men Movies . I mean Damn !!! How many more New Casts , actors ,and reboots are there gonna be ??? Are they gonna replace Thor for Thor 2 , or Replace or reboot Green Lantern 2 weeks after It comes out ?