By looking at people I know and their tablet ownership, I think you're the one mis-estimating.
But the psychology probably goes further than "I'll buy everyone in the family a tablet" and is more like "Well I was thinking of maybe upgrading soon in a year or two, and anyway I'm tired of loaning the kids my tablet, how about I buy that new shiny one for myself and the kids can have this one?" to just push people over a threshold of spending
Apple devices have so long lifetimes with iOS/iPadOS updates you can just cycle the devices among family.
I get a new one because I ca... because I need one, yes, that's why. And my old one goes to the kids. A few cycles of this and everyone has their own, and all of them are still officially supported with major version upgrades - not just security patches.
Their assumption may not apply to you, but I assure you that Mr. Apple himself has considered this possibility and allocated a significant amount of resources to finding the most profitable solution.
But the psychology probably goes further than "I'll buy everyone in the family a tablet" and is more like "Well I was thinking of maybe upgrading soon in a year or two, and anyway I'm tired of loaning the kids my tablet, how about I buy that new shiny one for myself and the kids can have this one?" to just push people over a threshold of spending