Movie franchises are Hollywood's favourite thing in the whole wide world. There's nothing more appealing to movie executives everywhere than a franchise - one that they can potentially exploit for decades, luring audiences back into theatres with the promise of a "bigger, better" movie in the series. This is made incredibly easy to achieve when the first movie in the franchise is considered to be a classic:
The Godfather, Jaws, Rocky - if they made another movie in any one of those franchises right this second, dubious fans would still head to theatres to check them out. But a franchise only really ever manages to succeed in the long-run if the original star remains attached to each and every installment, and even then it's notoriously shaky ground - nobody really wants to see a franchise continuing without the characters that audiences have come to know and love right there in the middle of them. When a Hollywood star decides that it's time to bail on a series, either because they've done all they can with it, or because it's too far removed from the original product, Hollywood should take note and call it a day. But they don't. And if the main star says goodbye, they'll just replace them... often to incredibly disastrous results.