why not shot on film?

The level of detail in film is way beyond 1080p. Decent quality film will resolve 150 pixels per millimeter. Given a 35mm film is on say Vista Vision format then you're looking at a filmed area of about 35mm by 27mm or 5250 by 4050 pixels vs 4k resolution of 4096 by 2160... Moreover your digital uses a bayer pattern so the actual detail on 4k is not even really up to the 4096 by 2160 since the actual pixel are extrapolated from the bayer pattern. Clearly the level of detail afforded film is still ahead of 4k video, if the restored trailers you are looking at look like crap it is more likely a result of either a poor quality film or low end film scanner used to capture the film.. In some case probably a combination of both.

So you saw some restored 35mm film on youtube and thought it didn't look that great. You're watching an analog medium on a digital medium. That's akin to me saying I watched an 8K Rec 2020 video on an iPhone 5 and it didn't look all that great. Well of course it wouldn't. The technology couldn't handle the full spectrum of the medium.

A good film print projected through a good film projector will spectacular. No matter how far you zoom into film, there's always information but you also introduce grain. If you zoom in far enough to 16K or 32K or whatever, there's going to be space, a lack of information, between pixels.

They both have their limitations. To a certain extent, it comes down to personal preference. I personally prefer film and I'm anything but a hipster. I've been conditioned, like many people from the 20th Century, to see film as a luxury because it was expensive to shoot on and thus the look of film was thought of as more professional. Movies like The Force Awakens looks like video to me instead of an expensive, Hollywood film. It gives me the feel of a local news channel affiliate on video which, in the past, was always viewed as cheap or having a cheap look as opposed to a filmed Hollywood production.

A 4K film scan of 35mm is a feast for the eyes. Digital just can't capture the fine subtleties of film, like grain and texture.

Yup yup I agree!
These two and a handful more account for the remaining 1% imho, while the above rant is my broad stroke sentiment about a trend / the current state of affairs. On BR it perfectly put across a quality of virtual plasticine artifice that speaks directly to the film's core concepts.

It's great that digital exists for a lot of reasons and there are times when it suits the material. That's really the heart of the matter for me, that the choice of medium should as often as possible be an artistic decision and have been well considered on the axis of form/content/meaning and gestalt. Digital has become the unquestioned default choice for non-artistic reasons today and it ends up undermining the material very often, and leaving us with a sweepingly heartless vista on contemporary cinematography. There's a sort of athleticism around the trajectory we're on with hi-def that has more to do with consumerism and technological chest beating than it does art. It's a feels bad man for me dawg.