it has problems with very light items.

If it doesn't work well on EVERY SINGLE ITEM in the store than the system is not designed well. Problem is that getting that last 1% of items to scan properly would probably increase the cost of the units or their software by more than a store is willing to pay. Your struggle costs them nothing.

Sounds more like companies refusing to pay a human and relying on robots and the honor system. Doesnt matter how heavy or light items are if a cashier is ensuring every item gets scanned.

The problems with self check outs are caused by the self check outs and corporate greed.

My local supermarket (one of the big two Aussie ones) has this neat technology

You can skip bagging things you buy, you can bag/weigh some things and not others, you can refuse a receipt

They protect themselves by having a staff member in the self checkout area