Then viewing distance and hence targetting distance could be individualized for each unit. This would in turn make helicopters more dangerous, since they could spot and attack units from much farther out. Admittedly, some tanks do have anti-aircraft weapons and some MFV's can use their main guns against aircraft, but only if they're close enough. Any smart helicopter driver would keep his craft out of danger and use his long range missiles whenever possible. But in Force 21 they can't use these tactics.
Helicopters seem to die too easily.
Since most units seem quite capable of defending themselves effectively from air threats, there's little reason to have specific anti-air units. Again, increasing the targetting distance would enable longer range SAMs to be used, and give a reason for anti-air units to exist. Ultimately, it is the shortened weapon ranges that result from the limited targetting distance that severely compromises the realism of the game. The final aspect hurting the realism of Force 21 is the lack of infantry. Surely it would not have been difficult to include some fire team units in the game, along with options to board apc's, entrench and camoflage, and the ability to move through or hide in a forest.