As a woman with long, curly hair, if I don't condition, I go from having nice curly waves to a ball of unmanageable frizz. I've also had to experiment with different shampoos to get the results I want, to clean my hair without damaging it. It definitely isn't engineered for marketing purposes. Ask anyone with long, curly, or kinky hair and they'll tell you how important their hair routine is. If you have short, straight hair, it probably doesn't matter nearly as much. Your hair is fully replaced in a few weeks. I need to keep my hair healthy for years. My hair is down to my sternum, and it took years to get this long. If I didn't take very good care of it with the right soaps and conditioner, it would not be able to be this long or this nice.

Reducing it to just chemicals and chemicals is well, reductive. Basically everything is a chemical. Your natural oils are chemicals. Sweat is a chemical. Dirt is chemicals. We need to wash out some chemicals, but we don't want dry scalps or damaged hair, so we need to use the right soaps and the right conditioner to remove those bad chemicals and replace them with chemicals to mimic our natural oils until they build back up. Most people probably don't need to wash their hair as often as they do, but conditioning is definitely not a marketing scam.