I think it definitely has something to do with monetary factors.
Rising ticket prices mean that the audience expects a cinematic event for their money. Like if you are spending as much as $100 as a group for tickets and snacks/drinks, would you be satisfied with a short 90min film or would you rather have a spectacle like avatar?
And from the studio side there are all the factors that also lead to the death of the mid-budget movies. You are already spending a ton of money on marketing and for the general audience it's probably a "winner takes all" scenario. The mid-budget movie with its similar sized marketing campaign can't compete with something like a Deadpool 3. And if you are spending big on marketing you might as well spend more to make sure you hit. And stuff like good CGI that is expected nowadays is also not cheap
Additionally larger overall budget+returns give you growth. the amount of days in a year is fixed, so there is only so many release windows that make sense without movies cannibalizing each others returns. Even if you can only dream of the margins of some low budget horror movies.
Not a horror fan myself, so I actually don't know if that's the case, but I could imagine that niche genres that don't need that much marketing campaign (and don't have a potential audience large to support a larger one) will still have shorter run times.