I could be wrong about this, but weren't heavy flaks usually around strategic targets like cities? A 128mm would have a really tough time
trying to track a low flying bomber or fighter attacking an airfield. Be much more effective attacking a heavy bomber formation attacking a
factory or city.
I am also not sure if the 88mm flaks at an airbase would have the AP rounds to engage ground targets. In real life, if tanks were getting
close enough for flaks to direct fire them most luftwaffe crews would be looking to get out of dodge