Thank you for the opportunity to help you with your question!
It's because hers, theirs, his, mine, etc are POSSESSIVE PRONOUNS. They never use apostrophes. It's simply a rule. For all other nouns, you must use the apostrophe to express possession. For example, The man's car. John's book. The police man's gun. Etc.
Please let me know if you need any clarification. I'm always happy to answer your questions.