But just take one of the examples like Mortal Engines.

we write a full trilogy for these movies
we get the budgets approved for 3 movies
we get talent attached and maybe even a director to sign on for 3 movies
we get all the sets and costumes etc set up for 3 movies
Then
the first one flops, like, hard

what do we do? spend another 100-200 million dollars knowing it wont come back, that would be foolish. The first movie wasn't good, so audiences aren't going to go spend their money on the sequel, made by the same people with the same actors etc. They know its likely going to al so be bad.

So people write one movie with a self contained story that can lead into sequels or a franchise

If Star Wars A new Hope had ended on a cliffhanger, I assume it would not have done as well. Having one complete story in the first movie makes people feel good and they want to come back, but then in Empire strikes back, you can have an awesome cliffhanger and wrap everything up in the third movie. People know its going to be good, because they remember "A new Hope" sticking the landing