The important question is why smartphones are designed around not having root access and computers are?

What are the incentives at play?

The answer is obvious, tech companies wouldn’t have given users access to root control on their computers either if they knew what they were doing and thought they could have gotten away with it.

It is just circular logic claiming smartphones have to be this way, circular logic that provides a rhetorical smokescreen for the process of corporations taking our agency away from us over our lives and the tools that sustain us.

You're free to install another operating system or variation on Android on your phone still. And if you decided to go with another Android such as Graphene, you'd still not want to root it because it's a security risk.

The issue is that you don’t want to give some random untrusted process root access. You, the user, have root access as long as you’re capable of running processes as root, but that doesn’t mean you should.

There could be tons of apps on the iOS App Store or Google Play Store that are completely benign under the existing security model but do nefarious things when run as root. No one knows that for sure because they aren’t tested under root by Apple or Google.

The problem with root is that it’s giving the process the keys to the Ferrari. That’s long since been decided to be a bad security model. Far better to have the process request permission to access particular resources and you grant them on a case by case basis.