Because there are two problems involved here:
Although the other party has not received the receipt, the other party's payment is usually evidenced, and the receipt is not the only evidence. Many physical evidence and witness can be accepted by the court. Therefore, if you want to maliciously obtain improper benefits, there may be risks of civil loss compensation and criminal responsibility.
Personal integrity issues. Honesty is a person's moral behavior, a citizen's second "identity card", and a general term for honesty in daily behavior and credit in formal communication. That is to say, treat people sincerely, be honest, keep your word, do what you want and keep your promise. Don't lose big because of small things.