Here's why:
* Wash is a transitive verb: It requires a direct object. You wash *something* (your hands, your clothes, the dishes).
* Reflexive pronouns are used as objects when the action is directed back to the subject: For example, "I hurt myself."
Instead of using a reflexive pronoun, you would typically use a possessive pronoun or noun to indicate what is being washed. For example:
* Correct: I wash my hands.
* Incorrect: I wash myself.
Exception: You *could* use "wash oneself" in a very specific context, like referring to a general act of self-care. For example:
* "It is important to wash oneself daily."
However, this is much less common than simply saying "wash your hands" or "wash your body."