When someone comes to your home whether invited or uninvited, shouldn't the person who lives there greet him or her?
Dear Left standing,
If you were not invited to "Jane's," but stopped by without any notice, then Jane doesn't need to answer the door.
To drop by someone's house unexpectedly is not proper etiquette unless it's an emergency. Even if you're family, you need to demonstrate common courtesy, and phone ahead to let them know you're coming and to see if it's okay.
No one is obligated to answer the door when not expecting someone. People's lives are busy enough these days without having unexpected company.