As ƒιzz's answer points out, you've got the wrong idea about how the damage formula works. You're assuming the offensive and defensive stats factor in additively, but actually they factor in multiplicatively. If they did work like you assume, one point of HP would indeed be exactly equivalent to one point of Defense and Special Defense, and there would be little reason to bother with the latter unless you've already maxed out HP, but that's not actually the case.
Specifically, though, I'm writing my own answer to explain a bit better when you want to invest effort in HP and when you want to invest it in Defense/Special Defense.
Basically, you can roughly simplify the damage formula as follows:
bunch of stuff / Defense
That's for the number of hit points of damage dealt, but actually what really matters is the fraction of your total HP that you lose. So divide by the whole to get
(bunch of stuff / Defense) / HP
which is equivalent to
bunch of stuff / (Defense * HP)
You want the damage to be low, so you want that divisor to be high. In other words, you want the product of your Defense and HP to be as high as possible. And due to the way multiplication works, this means you want your Defense and HP to be as close to each other as possible. (Think of how 20 80 is 1600, but 50 50 is 2500.)
This means you basically want to try to raise whichever is lower, which is usually Defense/Special Defense since HP uses a different formula that makes it higher than other stats.
However, do note that there are two defensive stats but only one HP. If you invest all your effort in Defense, then it won't help you at all against a special attack, while effort invested in HP will reduce the proportional damage from any kind of attack. Maybe you intend to use this Pokémon as a physical wall and will switch it out against a special attacker anyway, or something like that, and then the "raise the lower one" rule holds, but otherwise you'll probably want to be well-equipped in both defensive stats, which complicates things a bit since your effort points will be more spread out if invested in your defenses than in your HP. Overall, if you weigh Defense and Special Defense equally, you'll want to minimize 1 / (Defense * HP) + 1 / (Special Defense * HP), which works out to preferring HP being a bit higher (exactly how much higher depends on the precise numbers involved).
