Because fungal is the only ability in the game that is instantly cast, completely freezes units over a large radius, and does damage.
You're kinda grasping at straws here.
Not really, there is no difference in effect between the examples listed and fungal. If you walk into infestors with clumped units you lose them with no chance to correct your mistake. Same goes for banelings against tanks, colossus, etc. Why does it matter that the units take longer to die with fungal?
Tanks and Colossi don't freeze units in place, and both can only hit ground units.
Fungal freeze any unit in place except for Ultras, but no one cares about that because you can't claim ZvZ is imbalanced, and Protoss doesn't have Dark Archons for Mind Control, and if they did, Ultras have Frenzy, so it wouldn't matter.
If Thors and Archons also had Frenzy and were immune to the snaring effect of Fungal, people wouldn't complain, but then Zerg would complain about early Thor and Archon pushes because Neural parasite and Fungal wouldn't effect them.
Basically, the problem with Fungal is that there are no units for Terran or Protoss that can counter it.
FFs get crushed by massive units, and don't effect flying units. Concussive Shells again, don't effect massive units and can't hit flying units. Tanks and Colossi can't shoot up.
Fungal is the only ability in the game that can stop flying units in their tracks. Yes yes, split them. Except Zerg often get a dozen or more Infestors, so splitting is not effective...
Another solution would be to have Fungal not effect Massive units or not hit air units, but again, then Zerg would complain that either Archons and Colossi are too hard to deal with, or that ZvZ will turn into Muta vs. Muta, depending on what you go with.
But yeah. Basically, the point is that people wouldn't complain about Infestors as much if Protoss and Terran both had at least one unit that didn't get snared by Fungal and couldn't be NP'd...