Smartphones and tablets are trying pretty hard not to be general-purpose computers. iOS devices don't run arbitrary user-specified code unless hacked, and many Android devices are designed not to let the user have real admin rights (i.e. root).

This seems to be getting worse, for example Google and its financial institution partners have decided that general-purpose computing and payments aren't safe together. Google Pay suddenly started refusing to do contactless payments on my rooted phone.

You can use magisk to keep Google pay working on most phones still. (Works on my pixel 5)

I do use magisk. It started failing safetynet a few days ago, which I discovered as I was trying to make a payment.

iOS devices can _run_ arbitrary code without being hacked. It just stops working 3 days after being compiled so its good enough for kids to develop on but not good enough to distribute.

That's special purpose for the general public, the opposite of general purpose.