If we lived in a high trust society, you could just trust people to scan their own items and walk out.

Human nature probably prevents that from ever being a reality, at least at scale. In a tiny tight knit community where literally everybody knows everybody else maybe you could pull that off, but even then you have to get a bit lucky.

In a world where anonymity is a thing there will always be at least one inherent shithead who ruins it for everyone. Even if you do have a community where it's true, that can change anytime someone has a kid or someone moves in.

Most grocery stores I shop at have merchandise outside the front doors and between the doors and the registers. You could easily walk out with unpaid merchandise and no one would notice.

The Dash Cart was pretty close.

Can you name some high trust societies?