Because "Aircraft" have a notably smaller healthpool.
For a Squadron of 9 planes with 2,400 HP each:
2400 x 9 = 21,600 Squadron HP
21,600 x .00125 = 27 HP/sec
A Shimakaze:
The Regen is a little over half the Shima's Long Range AA.
It's a known issue, but the skill is functional enough to use even though it results in increased plane loss. For example, I run it on the Hornet since the B-25s can pad out of the extra plane loss while allowing for more aggressive positioning to mitigate the slow plane speeds.
We tend to avoid overhauling things unless we're doing several things at one time. Opening up the Commander Talent code is akin to opening up a car engine to do specific work. It's not seen as worth doing over something small when you could wait until there are multiple things to address at the same time to make it worth it.
Squadrons of 9 is the most common, though there are some squadrons of 12.
Hakuryu Torpedo
Midway/United States HE Bombers
Immelmann Squadrons (Torps and Skips)
CV Hulls do not get buffed. No Regen or Total Health increase.
Testing shows an expectation of around 3% damage increase in Arms Race. The buffs were continually reduced from prior iterations to find numbers which mattered for the mode but did not lead to massive distortions.
The Health Regen was reduced significantly to avoid massive changes in damage/battle outcomes. If the Shimikaze lives for 10 minutes after taking an engagement, 8.5 HP/sec x 600 = 5,100 HP Regen'd.
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