"Value" comes from reflection, and reflection takes time. A moment is much too brief for us to attribute value to it while simultaneously experiencing it in all its phenomenal richness/complexity. "Living in the present" means not letting that phenomenal richness/complexity slip away from our experience of Being in order to retroactively assign value (or lack thereof) to any one moment in time. Doing so causes us to lose the emergent moment (where the value of life is said, by some, to truly reside). That's at least one way to look at it, more or less.