From a google search: Cetaphil Gentle Skin Cleanser contains just eight ingredients: water, cetyl alcohol, propylene glycol, sodium lauryl sulfate, stearyl alcohol, methylparaben, propylparaben, butylparaben.
Sodium lauryl sulfate is a foaming agent, a surfactant that can dry the skin. No other particularly good ingredients. I'm not bothered about parabens, they're preservatives, but a lot of ppl steer clear of them.
CeraVe, for example, has 3 types of ceramides, glycerin and Hyaluronic Acid. Ingredients which are actually beneficial and work to repair and keep moisture in the skin.
I think it's a complete myth that dermatologists are, by default, up to date and experts on all things skin. Some are very much in the past, still recommend things like Aqueous cream, they will recommend what's on their list or what has been promoted by the huge pharmaceutical companies. It's not some huge conspiracy, but it's big business.
I've had hugely helpful advice on online forums like Mumsnet, bloggers like Gothamista and the Ordinary group - not for their products funnily enough, but Korean brands which seem to suit my reactive, eczema prone skin. I now know to look at ingredients and not the marketing claims.
All advice needs to be taken with a large pinch of salt. What is one person's holy grail can cause a terrible reaction for someone else.