Plus, it's tough to hang an Arriflex on a drone.
These days we're flying Alexa Minis with zoom lenses and full FIZ control on drones, but not IMAX or Alexa65 yet. One big limiting factor is the 55lb MAUW limit the FAA imposes. It's straightforward to get waivers for many of the new Part 107 restrictions, but the 55lb limit is less simple to do with the flexibility needed for film shoots.
There are certainly drones out there with the lifting capacity, but when you're talking about a machine that can fly a 70+lb payload, most of the time it's easier to bring in a full-size than get a one-off waiver and bring in a rare super-heavy-lifter UAV.