Why are defensive stacking mechanics like Swain's so weak?

Nasus stacks Q damage and eventually goes around one-shotting stuff. Veigar stacks AP and eventually goes around one-shotting stuff. Thresh stacks souls and eventually... is gradually stronger, I guess. Sion stacks health and eventually gains a Warmog's worth of health or so. Swain stacks health and eventually gains half a Warmog's worth of health. Garen doesn't even count as a stacking champ in this context (why not uncap it? it's not like the game doesn't have enough true damage).

Why isn't defense allowed to be as impactful as offense here?

I just played a couple bot matches: one as Swain and then another as Veigar, focusing on getting stacks. On Swain, that not only means hitting with spells, but hitting champions. Not only do your spells have to hit champions, only W and your passive count (immobilize with E or rely on an ally's CC, and then get close enough to right-click). On Veigar, you can CS with Q that hits two targets, or you can hit literally any spell on a champion.

In a 20-minute Swain match, I got 100 stacks, which is 500 health.

In an 18.5-minute Veigar match, I got 350 stacks, which is 350 AP. Let's extrapolate outward to 20 minutes and estimate 400 stacks, as the end of the match involves more teamfighting and thus more spells and stacks.

500 health compared to 400 AP. A Giant's Belt and a red gem - 1400 gold value - compared to two or three goddamn Deathcaps - 7k-10k gold value. And if Veigar actually buys that Deathcap, he gets 40% more AP from his passive. And that's fine! It's his thing. But health stacking isn't anyone's thing, even for champions who technically can do it. Even if we reduce those numbers to account for the fact that it was bots, I was trying equally hard to get stacks on both, so the contrast would be just as stark. Why does unlimited health stacking have to be so puny? Why does unlimited resistance stacking have to be only for armor and then compete with other stats (Thresh)? Does the current meta not have enough damage to deal with an extra 2-3k health?