I don't see the benefit to a major retailer chain to even buy insurance. Are we sure they do? Insurers would simply look at Walgreens' specific loss amount and charge them that same number plus overhead and profit.
It's mainly an accounting hack with little value to negative value for the company as a whole after your company reaches a certain size, I agree. There is the classic idea insurance idea that because retail runs on thin margins, keeping the money flowing is important so the whole house of cards doesn't fall down from a huge decrease at an inopportune time, but the relationship stays around way longer than is necessary because of the social organizational effects above, IMO.