Because given their arc and efficiency, I do not consider them problematic. It's the neutrino signature, and I would prefer them to stay as they are. The problem is more (in my opinion) that they never have a vulnerability window: their fast flux dissipation and small flux pool makes them vent super fast, witch means they are almost always firing at full strength at you. And given their armor, you have to fire a lot at them while sustaining heavy damage or shield strain.
So I consider their shields balanced. Problem is, everything else is stronger.