I think it’s how convention started, but then it gotten live of its own, cause all writers saw transformers in their childhood or some shit. Maybe some film-knower will correct me that it was due to some 50s classic :shrug-outta-hecks:

But also it makes “engaging” entwined narrative, when you have like 3 parallel lines of action, you want them each cliffhanging on top of one another. Lots of modern films have at least two perspectives if not more