Sequels also cost a lot because they don’t re-use all the much. The old way of working involved a lot of recycling (although it wasn’t called that). Props and costumes would be stored and re-used on multiple productions, whether sequels or not.
But now, each film sees a redesign of the costumes, new props, new sets and locations. Even when films revisit places they’re often rebuilt from scratch because the old set was torn down to make way for the next film to need the studio space.
It takes a real commitment, something like a Harry Potter series, to build sets and props to last. On most films they’re thrown away before the paint has time to dry.