Stores sell them because people buy them, and because manufacturers provide them. A store that doesn't provide cable locks loses sales on profitable accessories to stores that will carry them.
People who buy them do so because either they (1) are misinformed as to their effectiveness, (2) have a situation where a U-lock would not work, (3) prefer the form factor, (4) make the informed decision that their bike will be safe enough.
I use cable locks to lock my bikes to the wall inside my garage. Someone could easily defeat these locks. But that would have to be someone willing to be recorded by my Ring driveway cam as he approaches the house, and willing to make some noise in the opened garage cutting the lock, all while hoping I don't come through the door a few feet away to resume mowing my lawn.
I also carry a short, fat cable lock when I go to the grocery store out in the suburb in which I live. The grocery has a bike rack adjacent to where workers are constantly pushing carts back into the store. The cable diameter is 5/8ths in thickness, and about two feet total length. Given its size and weight, it may as well be a U-lock, but I bought it in 2001 before I knew that cable locks were inferior to U-locks. I probably wouldn't buy it again. But I own it, and it works well enough for my use-case. I feel safe enough using it for a half hour shopping visit when parked in a high-traffic suburban area.
When I commute to work I lock my bike up in a key-card access cage along with a couple dozen other bikes. Inside the cage we each keep heavy U-locks just locked to the racks. Since it's always there for me, I don't mind that it is heavy and uncomfortable to carry. Additionally, I leave my bike locked up in this cage for 8-9 hours a day. And it's located downtown. In this situation I wouldn't use a cable lock. The security measures at play here are (1) A security guard who walks past and scans a QR code every 20 minutes. (2) A chain-link cage that requires the use of a key card to enter. Only people who have paid for bike storage have key cards to this area. (3) A heavy duty U-lock.
So I do think there are uses for cable locks. I agree they are not as secure as U-Locks. But I keep seeing youtube videos of people defeating U-Locks too. At least with a cable lock I know all I'm doing is discouraging opportunists. I'm not thwarting serious criminals. But neither is that U-Lock.