* Items "falling off the truck", i.e. you expect 100 units to be delivered to store, only 90 were delivered. Workers rarely check counts while receiving in store backrooms, so retailers think these were stolen from the shelves;
* Employees stealing stuff directly, either from the backroom or from the shelves;
* Employees working in conjunction with non-employees to steal, e.g. sweethearting (https://en.wikipedia.org/wiki/Sweethearting);
* Plain old shoplifting, e.g. somebody swiping a few items from the shelf into their pocket;
* Organized Retail Crime (ORC): People coming into the store and brazenly stealing entire shelves or racks of merchandise.
The first four has been around since Retail was invented, but their impact is relatively small on a store basis. It is the last item that is the killer and what is referred to as the rise in "shoplifting" recently. This is not shoplifting in the traditional sense at all. As an example, consider Cotopaxi closing their flagship store in SF: https://sgbonline.com/cotopaxi-closes-store-in-san-fransisco...
Retailers are very reluctant to deal directly with the problem, i.e. stop the people walking out. The reason is that the usual profile of people engaged in OCR as well as its relation to various progressive causes makes this a taboo subject in the US. This also makes it hard to discuss the problem. Just the other day I heard a segment on NPR saying retail theft has not increased and "statistics are hard to come by".
Edit: For those who (rightly) point out the fact that retailers are doing cost benefit analysis and deciding that the bad press is not worth it: consider walking into a bank in California and stealing (without using a weapon, just intimidation) less than $950 (lower than the misdemeanor theft limit for retail) and see if the bank would do a similar analysis.