And the sad thing about that is, if anything, the games should've made your characters more vulnerable to high amounts of damage to compensate for the crummy AI (especially in SF1). If the bulk of your force isn't taking considerable damage from attacks, it should be to compensate for things such as extreme enemy AI, numbers, etc.
The problem though lies in the balance between stats of all your characters, and the designers taking your weakest characters into account. They wanted to make sure that characters such as Tao and Hans (I can't think of any super squishy characters in SF CD off the top of my head because I've only ever beaten that twice, with both of those playthroughs being years apart, and SF2 didn't really suffer quite so significantly from lousy character balance as SF1) could survive at least one hit in the end game with their 20-30 HP, and almost equally as lousy defense. While that's all well and good, the problem then arises from characters like Guntz and Bleu who end up with excess of 60 points in each defensive category, making them essentially immortal for the sake of weaker characters to be viable.
I haven't yet truly beaten SF3 because I'm still waiting on the total completion of the translation, but at least SF2 had better character balance, like you said. The low-to-high character gap between end game defense stats in that game was closer to about 50-70, whereas in SF1 it's more like 20-60.