Prior to World War II (WWII), soaps were actually soap. During WWII, the fats and oils generally used for soap making were diverted to making explosives, and so new ingredients were needed to make cleaners and bars. At the time, petroleum oil was everywhere, and inexpensive, so petroleum derivatives were used to replace the natural oils and fats previously used to make soap. Because petroleum chemicals were incredibly inexpensive, soap making companies continued to use these ingredients long after the demand for explosives ended.
Petroleum derivatives in commercially available detergent bars (these products are NOT the product of an oil/lye reaction and cannot technically be classified as soap) include known harmful substances including parabens, sodium lauryl sulfate (SLS), and synthetic fragrances. Many of these compounds have harmful and toxic side effects, and some carry reproductive toxicity. Most of them have never been tested for safety in the first place. We’ve just accepted that they’re part of life.
Detergents will strip the natural oils from your skin, leaving your skin dry and flaky, causing you to need to apply a petroleum-based moisturizing cream to “fix” the problem. Soaps, containing natural oils, generally are milder and do not remove the natural oils from your skin in the same way.