Total security does not exist, as the saying goes. These are several counter-measures that emerged in the social sphere of common people, to increase security a few levels and hopefully avoid the worst scenarios at least. I don't think almost anyone has all 4 smartphone categories i listed, but if you ask around, most would recognize the measures you are talking about.

A disaster scenario in practice means you are screwed either way here, tons of physical documents can be lost or destroyed too, but that is a calamity that by definition is exceedingly rare.

Most are worried about common thiefs, a scenario of robbery or assault that can happen anytime anywhere and frequently, repeatedly. The overall threat here is far larger that the rare scenario.

Now, to the complexity. Usually the thief smartphone is some old phone still around, might not even be working, and no one puts anything in there, it is literally a throw-away device. It has no complexity or hassle, and usually is free. The house smartphone, people usually repurpose some old smartphone or buy a cheap android. I still have an iphone 6 for this purpose, and if the app is up to date, it usually is safe enough. People will not be using the phone for anything else, it will not leave the house, so exposition is severely reduced.