Why is cheating morally wrong?

Cheating morally is a wrong action, because cheating is, as one case of the definition of immoral action, especially in the case of your question;” cheating in tasks in such a way to give you an advantage over others.”That clearly disregarded right of others.

Is cheating really wrong?

Cheating is not only a violation of trust, but also a betrayal of values. ... If you've been cheated on, Marin says, it doesn't mean that there's anything wrong with you. Though cheating often involves sex, it's rarely about the sex itself. It's more about having someone new providing attention.