There's likely got to be a better reason for you. Maybe even for me. But the reason at the studio is that there's a rock-solid business plan. There's no other type of film where you can predict the box office with such certainty. I think 60% of the take from the prior movie is considered the floor for a sequel, with 50-60% outgrossing the original. The previous film is the marketing anchor for the latter, and that saves in marketing costs. The stars of the previous one are not only now more familiar, but you can get past any worry about testing them in the roles, as the audience has already accepted them in the slots, and if you had any foresight, you signed them all to a three-picture deal, with options on the latter two, so you even though they are now more famous due to a successful initial film, they still cost you what they cost you before the first film. If the first film bombs, you don't pick up the option. Usually, there's no buyout fee for that. On top of that, production costs are additionally controlled because the prior production is your blueprint. Lastly, you own the property, so you don't have to pay the rights owner again. If the original screenwriter or director wants mo' $$, you can toss them and hire some hack. The style is already established from the first film. So controlled costs, a mostly guaranteed take, a proven production blueprint, and unlike non-franchise fare, you don't have to worry if the critics think it's derivative, because duh, it's supposed to be. It's certainly a slow-motion race to the bottom artistically, but it's sadly good bidness. Not a lot of execs lose their job for greenlighting Deadpool 3. And when you've got a three-foot pile of cocaine in your rumpus room, and Jennifer Lawrence in your rolodex, you probably make a lot of decisions based on what's not going to get you fired.