I learned this as a kid working at a grocery store asking the same question. When you leave your cart by your car instead of returning it, it takes labor to collect the carts around the parking lot. That labor costs money to the company. That money gets passed on to you as a customer buying products.
The 30-45 seconds it takes to return your cart may seem inconvenient. Rinse, wash, repeat a thousand times in your life. Multiply that by millions of people nationwide across all grocery stores. That adds up.
Stop being lazy. It’s not hard to return the cart. If we all did it, it would passively reduce labor costs. That could potentially reduce price hikes on food.
It really isn’t about optimism. I know prices would go up no matter what. But it does allow a bit more passive control without being a twat about leaving carts out.
I wasn’t aware of the shopping cart theory. Thanks, now I have a name for my belief. :)