If lethality is such an issue then why aren't we just stacking HP

because you don't stack hp to counter lethality, you stack armor. lethality is just flat armor pen. If you have 5000 health with 50 armor, and the varus has 50 lethality, he's doing true damage and will melt you in a few hits. If you have 3000 health with 400 armor, and the varus has 50 lethality, he's doing 22% damage to you.

because you'll still get eaten up by lethality champs if you have no armor

especially if they have a fucking execute