In addition to being loaded with salt (despite the current trend of handwaving salt issues away, salt is a problem), they are often tenderized beyond recognition, have added preservatives, flavor enhancers, fat, and sugar. You might be OK with those things but it is absolutely not the same thing as a regular chicken you buy and cook yourself -- THOSE are the ones that contain just chicken.
How the chicken is processed depends on the supermarket. At many places you can just ask them what they do and they’ll be glad to tell you.
That said, brining chicken prior to roasting is very common. I do it myself and it is almost essential for free range chicken which tends to have more blood near the bones, brining helps mitigate that. The other stuff is more or less just spice rubs. YMMV, depending on your supermarket. Roast chicken tends to go fast so I would be surprised if they need to use preservatives.
There is absolutely nothing wrong with flavour enhancers. As for the other things, you are varying degrees of correct (for example, it is easy to eat too much salt but not if what you eat in a day is made by you otherwise). The sugar is obviously not ideal if you're going for low carb, but thankfully you can check this by reading the ingredients list and nutritional info. There are no mysterious things going on here.
