Turrets must have extremely short range compared to an airbase. Airbases would be one of the longest-range strategic assets you can have, outranged only by heavy artillery and strategic nukes. A turret is a ring around itself protecting its immediate vicinity only. And more importantly, aircraft would still be stopped using anti-air, especially fighters. Unlike turrets they suffer force depletion. Lastly, and perhaps most importantly, the existence of numerous vulnerable support assets like airbases and artillery mean that it is more important to stop enemy units from reaching your rear echelon than it is to fight in a blob with maximum force concentration. Even a small enemy force in the right, sensitive spot, should be able to do severe damage to powerful units that can't really defend themselves effectively. And, by the same token, even a small force of your own can do a ton of damage to a larger enemy force by relying on the help of those same supporting strategic assets such as artillery and airbases. One scout bumping into a large enemy army can call in a rain of artillery just as well as a huge force, so overdeploying forces can in some contexts be just as bad as underdeploying. The "line of battle" where you are controlling territory with a clear front line, with securely controlled territory behind your front line forces, no-man's-land, and then enemy front line forces and the enemy's rear echelon, is generally not how PA works. Generally in PA your forces fight at the highest possible density so as to win a fight against an enemy force doing the same. The "blobs" become a gameplay problem when they become very large. A huge blob should be split into several pieces with each small group positioned in different places, each relying heavily on shared indirect support (artillery, air power, strategic weapons, etc.) which are likely grouped together to make them easier for troops to defend. Still, on huge planets, you are likely to need several such installations simply due to their range limitations.