In which case you buy a smaller quantity elsewhere. I don’t know anyone who shops exclusively at Costco. Most people buy large quantities of the things they use a lot of at Costco, and also visit grocery stores for other things. Besides, most of what they sell has a long shelf life. With the exception of their very limited produce and Dairy, just about every other perishable food they sell is freezable.

The people I know who shop at Costco aren’t throwing away half of what they buy. They are very often families that are actually pretty efficient about using what they buy. Big families, restaurants, remote work camps (I live in Canada) are the people I see completely filling carts and SUVs at Costco. For them, shopping at Costco is a way to avoid waste in terms of small packages and multiple small trips.

While there are certainly people that shop there and waste what they buy, it’s a pretty overused exaggeration to say that it is any more than a small fraction of their buyers. If you want examples of frivolous consumption, a barebones warehouse store selling staples in bulk is kinda the opposite of that in many ways.