FOMO for sure is one of the driving factors.
"We cannot risk looking outdated". So weak management, probably.
But also talent availability I suppose. If there's a new trend, the pool of people you can hire include many that are in on that trend.
UI frameworks too, probably. The modern thing™ does the modern thing™ and you do want to be on the modern thing, because you fear that only that receives security fixes or whatever.
I really don't think that "keeping people entertained" is a sensible goal within the context of building software as tools and not software-adjacent Art.
Which is not to say that I would not want a great and polished experience, but that is not equivalent to "being entertained".
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It would be nice if not everything one interacts with would try to get some sort of emotion out of me. Bring back being bored.
I don't think I know any non-tech people who like things changing about. Some tech people like that (I don't), but for the non-techy, it's just another thing they have to relearn for no good reason.
As opposed to what? Be entertained by all the bells and whistles of modern operating systems that have practically unusable user interfaces?