It's a value for the money consideration, not an economic one.
I mean, one hopes that economics is largely based on that, yes.
In the grand scheme of things, $15-$20 isn't a lot of money. Most people can afford the price of a ticket. (Many people will drop $1,000 on an iPhone and not even blink).
Comparison to an iPhone is sketchy, at best, as you are comparing 2 hours of entertainment to a durable item you use every day that you can expect to last you at least a couple of years.
$15-$20 isn't a lot of money. But that's per person, not counting concessions. With concessions, it is more like $60 total for a couple, and $120 for a family of four. And yes, it becomes a value for cost question - but that question is asked
in comparison to other forms of entertainment.
As in, "I can pay $60 to watch this movie now, or I can watch something at home on a streaming service for a fraction of the effective price." Or play videogames for a fraction of the effective price. Or play a boardgame for a fraction of the effective price. Or read a darned book for a fraction of the price....
Oh, and that movie will come around to the streaming service fairly soon anyway, where they can see it with cheap soda and snacks. What, exactly, then, is the pressure to see it in the theater?
This last is a reality that studios have to face - covid forced people out of theaters, and drove them to streaming services. And, now that they have those services, they don't feel so much of a need to go back to that theater, in general.
The quality of the movies hasn't actually changed. We are just comparing the increased cost of the theater experience to the low cost of the home experience. If, for the very same film, the theater experience is insufficiently superior to justify the expense, then ticket sales will suffer.
The issue is whether people want to spend their time and money on something they may not like. (e.g. if you went to two Disney movies and didn't care for them, you are less likely to spend money to see a third one.)
I think you are missing the reality I explained above. It isn't that movies are bad today. It is that the
theater experience is not so much better than the home experience to bother. The
need to see things in the theater has largely dissolved.