Two reasons mainly. First, if there are any brushes at all in the car wash, it’ll scratch your paint right up. You will have swirls in the paint that show up quite vividly especially in a black car in sunlight. Second is that the chemicals they use in those washes are very cheap and will eat away at your paint or any finish (short of full PPF) and your car color will eventually start to fade. In a year or two, line it up next to another car same age but they used proper wash materials and techniques (or took it to a service that did) and you will see a pretty significant difference in the colors of the cars. Like a black shirt after several washes starts to look more grey than black.
