Planned obsolescence is half of it, and the other half IMHO is this, people now want to replace their appliances about every 7 years.
I sold my moms 53 year old house with the original wall oven, stove, furnace and a/c. The "flippers" who bought it ripped out the kitchen, so the oven and stove were placed, but when they put it back up for sale, that ac and furnace were still there.
My mom was, however, cheap when it came to washers, dryers and refrigerators. She bought the cheapest Kenmore model from Sears, usually got 20 years from them, when something acted up, she just bought new ones. She NEVER paid more than $200 for a washer or a dryer.