My understanding is that it's about loss prevention.

They think that self checkout is a high risk for shoplifting, so they want it to be a manageable amount. They need confidence in their security monitoring strategy before they go all in.

After a long trial here including employees monitoring and AI monitored video, the store near me now has maybe 30 self checkouts, with 10 having nice big bagging areas. I never have to wait in line anymore.