so when they talk about length, they’re talking about the physical length of the film. 35mm film refers to the width. depending on the format being used- super35 for example- the length of a single frame or still on that film is about 18 mm.
as for why film- because digital recordings are actually pretty heavily compressed and once you lose that data, it can’t be recovered. the information stored on the film is actually much, much more densely encoded even if it’s analog. additionally, there’s some effects that simply can’t be perfectly replicated using digital, simply because it’s a fundamentally different medium.
You’ll have adherents to both camps, but ultimately it comes down to what produces a better film, and for box office productions, that’s still analog film.