Why do people act like friendships will fill the need of a romantic relationship?

I see this a lot around Reddit. Someone will make a post about being lonely, and wanting a partner (usually a girlfriend). There will always be multiple responses from people telling them they need to focus on their friendships before they even consider getting into a romantic relationship. Friendship is great, but even the closest of friendships won't fill the need for romantic love. Why do so many people act like they are one and the same?

Honestly the opposite applies as well. A close romance won't make up the need for a good friend.