[Mobhack]

Fortifications have their use in early WW2. And on up til when ATGM and 100+mm tank guns are common in MBT - then they are one-shot wonders. Which is why modern armies dont really go in for such any more, unless using HESCO against small arms and mortar threats from insurgents.

The bunker classes aren't primary AA classes. A few OOB designers though plonked FLAK in them for their own purposes, probably primarily anti-tank use?. You would have to ask the original designers as to why!. Bunkers basically fire AA in self defence only - and unlike AAA classes, cannot have EW values so any SAM in them are particularly pointless.

As Scorpio said, an ordinary AA gun, or even SPAA vehicle, which will be dug into an emplacement for free in the defence, is what you want if AAA is your goal.