Not marrying and trust each other? Nah, I think they don't marry because they don't trust themselves.
Marriage means stability, considering the other party as your responsibility. Being toghater for the sake of being toghater won't last long. Life isn't long but isn't that short either and feelings change, so being a couple while dating each other is not stable and secure at all. It's just based on fluttering feelings when life is much more wider than that. It's scary.
You are not a family and you can be trashed anytime while not having the right to voice your opinion about that and yes, no one will understand. Nao is a real example here. Long relationship for 7 years without marriage is like taking advantage of the other party. There is no respect in these kind of relationships imo.