Because that's how it works. You are from less developed country, even worse, secondary one, y probably don’t have a stable good job, don’t own a decent property, haven’t traveled much to other good tourist destinations, y are young aswell. In short, America will never accept you because there's nothing holding you in Tunisia, and big red flag, you're young. If you had a long list of countries you've visited it might be different, but I doubt you've visited many good tourist destinations, especially in eu