[2020] If magic damage is not stronger than phisical damage, how come full ap comps are not bad while full ad comps suck?

I always see people saying that magic damage is fine the way it is and that magic resist needs to be low for the game to be balanced, they say that because aas scale with ad and they can crit armor needs to be a lot and it can be stacked up to 400/450 if a tank only builds armor, while magic resist can at most reach like 200/250. Another proof for this is the fact that Ap champions counter tanks, having so much penetration in their normal core items, not needing to build some sh*tty inefficient item that gets to be worth about half its cost.
I have no problems if a mage oneshots me if i don't build defences against them it makes sense, the problem is when a tank invests gold to get something that does not really pay off, sometimes it's just more worth to straight up build hp instead of magic resist.

Having armor being balanced around 100% crit 500 ad Aphelios and magic resist balanced around full ap Garen does not make sense, and if someone thinks otherwise I want to read their reasoning, which can possibly change my mind, i don't deny it.

I'm not biased since I don't play tanks, and I actually despise them, but it seems unfair for me to see tanks blown up by someone just because he happened to have magic damage and ap scalings.

I mean, let's say the enemy team has Vladimir top, Ekko mid and Brand support, what would be a good pick into that and/or what would a tank (Say Maokai or Mundo) build in order to negate those champions as much as a full armor Rammus/Malphite negates Ad damage dealers?