I've been washing my itchy, sore, scalp with dandruff shampoos and dermarest shampoo. my scalp still itches. why?

Doesn't change: the underlying condition. Most of the causes (allergy, eczema, psoriasis) have inflammation and the over the counter shampoos are not anti-inflammatory. Frankly, the prescription ones are quite weak also. Usually oral medication is required.

Dermatology: If it's bothering you that much, and what you have appropriately tried has not worked, consider a dermatology consultation. They may be able to come up with a diagnosis, and treatment plan. Hope this helps!