Why aren't any wingsuits able to sustain flight? I'm aware that under zero power it is a glider, but there are gliders that sustain flight and have enough lift to climb etc (Edit: this is not always true. See below answers for why). So, would it not be an interesting idea to make a wingsuit that you can use to climb and not just fall? 

With longer wings could it be done? Or maybe a longer airfoil?
I'm aware that under zero power it is a glider, but there are gliders
  that sustain flight and have enough lift to climb etc.


This statement is a bit incorrect. Gliders don't climb like aircraft as they are constantly falling to the ground unless they catch a thermal, wave lift or ridge lift, in which case they can ride the updraft to gain altitude. In theory, a wingsuit could do that if you could maneuver to stay in the thermal but wingsuits are not maneuverable like a glider and tend not to have nearly the glide ratio of a glider so they sink far faster than a thermal could cary them up. 

There is an interesting discussion on it here.