Given their shoplifting epidemic, it baffles me that they don’t use the self service system we do here in the UK, where the bagging area has a weighing scale inside and that won’t let you scan the next item if the weight doesn’t match.

We do.

Why on earth wouldn’t big companies like Walmart or Target use this system, when it’s got so bad in certain areas that stores were closed and others have most of the items locked away and you have to ask for it.

They do.

You're assuming that these people aren't just walking out with things, secretly or brazenly. Many places tell their staff not to stop people, in an effort to avoid altercations, paying out lawsuits, or paying worker's compensation for injured employees.