Franchises have been Hollywood's bread and butter for decades now, and essentially a license to print money; if people vote with their wallets and make a movie a box office hit, then it stands to reason that they'll come back for more of the same.
The studios find it easy to churn out follow-ups every couple of years, confident and safe in the knowledge that their target audience want to see their favorite characters again and again. This may be stating the obvious, but the real challenge is making them good on a consistent basis.
Star Wars, Terminator, Alien, James Bond, Batman, Fast and Furious, Planet of the Apes, X-Men, you name it; even the biggest and most popular brands in the business have struggled to maintain a high level of consistency throughout their existence. They all boast some fantastic movies, but there's also a couple of duds thrown in for good measure.
On the other side of the coin, plenty of franchises begin with a bang and start off with a great movie, only for the law of diminishing returns to set in. Sequel after sequel arrives, and yet none of them ever manage to recapture the essence of what made the first installment so enjoyable in the first place, until they end up as nothing more than a cynical cash-grab and a shell of their former selves.
I don't social network, so like or follow me in person but please maintain a safe distance or the authorities will be notified. Film and TV graduate turned chef, once labelled a misogynist on this very site in a twenty paragraph-long rant for daring to speak ill of the Twilight franchise. Go figure.