Taping drywall is an art form. It takes practice and a lot of patience. That's why taping and finishing drywall is not something you do on a single day off or weekend. It's something you do in 3 or more days depending on the size of the project.
A properly finished wall should not have any joints or nails visible, and achieving that perfectly smooth wall can be a challenge that many people would rather to leave to the pros.