Two adults, one upright full-size American standard fridge/freezer plus one large deep freeze chest freezer, both energy star efficient models; since I usually only go into the chest freezer once or twice a month to replenish the upright freezer, we probably spend a lot less than the advertised $27/year on electricity to run it & the food in the auto-defrost upright freezer doesn't suffer from freezer burn. We rarely (twice since the pandemic, usually more like 4-5x a year) eat out; the grocery bill for the last 12mo averaged $112 USD per person, alcohol & housewares like toilet paper included, partly by eating significantly less meat than most Americans & partly by butchering it all ourselves. I batch cook almost everything, freeze semi-prepped ingredients like cooked beans or chopped garlic/ ginger, buy huge quantities of fruit in season to prepare & freeze to avoid out of season pricing (& embedded carbon costs) later. Almost nothing goes to waste. Trimmings get composted, we use approximately one kitchen trash bag for the whole house's trash every four weeks, there's rarely enough organic matter in it to ever smell.
The main reason I think additional fridges caught on is, as others said, someone will be remodeling & then move the "spare" to the garage. Resale is cumbersome - American units are large, heavy, hard to haul - & square footage is comparatively cheap. (If an American can solve a problem with money, they usually do.) But then there's something else driving it, which also spurs the popularity of chest freezers:
American shopping is NOTHING LIKE European shopping. Car culture happened before many of our cities went up, sprawl is very real, you could be miles from a grocery on a residential street: a great deal of the country outside a few urban centers can't walk to any actual food markets (even if they want to), & even fewer are habituated to the idea. Cities were segregated on racial lines & often this meant intentionally making walking (read: poor) transit between neighborhoods difficult, effects of which persist today; markets tend to be where cars are. Making a few big trips makes more sense than driving off to a shop twice a week. Plus most the food is lower quality - underripe fruit, factory meat, bland veg, battery-cage eggs that were washed & must then be refrigerated, everything being optimized for cost & distant transit - which means people buy on quantity-to-cost motives instead, usually through big box stores. Much of the high-end produce is ultra-packaged & that packaging takes up twice the space the food would otherwise. A surprising lot of middle-class people still view farmers' markets with vague suspicion, as if un-plasticked food were unsafe or the vendors might slip copies of Das Kapital in with the eggs to indoctrinate their children, & there's such a legacy of fear that commercial activity in residential areas might attract unwelcome outsiders that it's frequently prohibited by zoning laws. Neighborhood markets are pretty limited to urban residents, often the quite poor & the very wealthy.
Counterpoint: food is truly much less expensive here, especially animal protein & dairy; if you want to live cheaply, it's quite easy to remain fed, or overfed. (We just pay it all back fivefold in healthcare.) I have the grocery budget pretty dialed in by buying enormous quantities of staple ingredients I prepare myself & knowing how to use spices, but if I want to eat a variety of greens beyond the romaine hearts, cabbage, iceberg, & spinach to which the big boxes limit their offerings, I either have to pay several dollars more a pound at a pricier grocery I don't otherwise drive to, or grow it myself (which I'm attempting to get back into this year.)
