They joke around with the possibility of being together. They say that they can't be with you... right now, because they have some things or the other going on in their life. The 'right now' bit of the sentence keeps you stringing along for a while longer. They may even throw in deals like "...by the time we're 40 and still single, we'll marry each other".
They want to be with you on their terms. You're there only when they need you. They treat you like a spare tire or backup plan, which mostly means that you come into the picture at the last possible moment, that too when their first choice has failed; for example, when a date to a wedding has bailed on them. But that's as far as the commitment goes.