Why didn't Jacob and Anna in the movie Like Crazy marry before Anna's visa was about to expire?

Anna didn't know if she stayed in US after her visa expired, she couldn't come back again. And they weren't so sure about each other. They love each other. However, marriage is a huge thing. They just graduated, they have their careers to worry about. Marriage is just too soon for them. And, I think Anna loves Jacob more than he loves her during the entire movie. Therefore, Anna takes the first step everytime. The best scenario would be they stay together and get marry when Jacob was ready, but it didn't happen.