You're not granting the assumption. Of course, on the premise of randomly assigned defenses and your opponents only focusing on one weapon type, defenses will be worth 1.3 hp/point. But neither of those premises are going to be true for a human player and an AI opponent. Human players should not randomly assign defenses, but even if they did, AI opponents tend to have at least two of three weapon types in their fleets.
Yeah... you're gonna hate this, but they're still worth 1.3hp per dp.
You have 10hp, 4pd, 4 shields and 4 armour.
You opponent has 1 of each type of weapon.
The enemy fires his laser at you. Your effective hp is 26 (10+0+0+16).
The enemy fires his missile at you. Your effective hp is 26 (10+0+0+16).
The enemy fires his kinetic at you. Your effective hp is 26 (10+0+0+16).
Your pd value is (16+0+0)/3 = 5.3. 5.3/4 = 1.3.
Your shield value is (16+0+0)/3 = 5.3. 5.3/4 = 1.3.
Your armour value is (16+0+0)/3 = 5.3. 5.3/4 = 1.3.
The ONLY case where it's worth more is if you're opponent is using only one or two weapon types, and you only defend against those specific weapon types. As you noted, the AI tends to use 2 or 3 weapon types in a fleet, so you're pushed into a position where you're getting an average of 1.3hp per dp.
The assumption that you want me to grant is that the very, very specific circumstances in which you're correct are the case. That's not how statistics works.