You're putting about a billion tiny scratches in your paint
OK, even if you forget about the acid for a second, you're still hurting your paint every time you go to an automatic carwash. Remember those horrific giant spinning monsters from decades past that would maul your car to get the dirt off? Those are mostly extinct today, but even the "softer" brushes repeatedly slap against your car and drag across its paint, carrying with them a bunch of dirt, not just from your car, but from the car in front of you, and the one in front of it, etc. Each particle is essentially a tiny piece of sandpaper eating away at your car's paint.
Even water pressure can cause damage
For the sake of argument, pretend you have a small rock chip. Maybe it's brand new and you've never even seen it. A quick blast of water at close range with enough pressure is all it takes to do serious damage. If your paint happens to get pressurized water under it, it can force up the exposed edge and peel away even more paint.