It's not possible to have a relationship with someone in runescape. But, it's possible for someone to pretend to be someone else online, and lie about who they really are.
It's not that running in the runescape game is more effective for establishing relationships. It could be advantageous for certain individuals. When you are being asked to meet you might be feeling more stress. In Runescape however, there's no pressure since nobody is expecting anything. Runescape provides minigames and other activities which let you get to know your opponent more thoroughly.
You are forced to communicate the other runescapers since you're always online and you have the chat going. You can do exactly the same thing in real life, however. This is what people do in real life, when they want to get to know someone of the opposite gender They request if they would like to see the movies.
It is possible to imagine yourself as a runescaper battling another player before they request for your help in any way. A good example is when a player might need to assist an opponent in the wild. It is not like a premeditated plan for a date, or going to the movies. This way, you can meet the other player. Which one would you rather to have? A relationship where you are able to discover more about the other player or one in which they attempt to impress you?
Usually when you meet somebody for the first time, there is no reason to lie. If you're truly in love with someone of the opposite sexual orientation, you can be able to lie to win their love. Do you really need to lie to someone in Runescape? Who you meet in Runescape won't be there in real life. They are in a different country. So why to lie? You'll never be scared of sharing your personal information, unlike in real-life.