Your specific point you seem to be hanging onto is why do the billionaires not have phones whose technology absolutely crushes your personal phone? I don’t understand why the question is interesting, but here is why they don’t:

What would they do with it that other people cannot do for them, faster and more cost effectively?

The limitation on personal productivity is not the pocket computer, it is access to computing resources. Right now the closest thing we have to a competent human, which can be easily had for $50k/year or $2M/working life, costs around 500 times that, lasts 1/10th as long, doesn’t work unless I’m talking to it, and isn’t nearly as capable.