The Big Box stores are getting some of their customers for commodity items.
And the customers are
1. installing them,
2. removing them because they are defective,
3. returning them to the big box,
4. checking the new one,
5. going back home,
6. reinstalling it,
7. and then bragging about how much money they "saved" by DIYing it.
If they bought it and had it installed by a professional, then doing steps #2 through #6, will create additional charges for labor while waiting for it to happen or coming back a second time.