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.


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.