Muh "Tankbusting".
Battlemages need to tankbust because they're short-range and slow-moving, and can't rely on getting to the backline.
Juggernauts need to tankbust because they're short-range and slow-moving and can't rely on getting to the backline.
Assassins need to tankbust because their delayed burst means they can't reliably kill the backline before dying to the backline, and have therefore been largely balanced around shredding any isolated target, regardless of tank stats.
Light fighters need to tankbust because they need to be able to force kills on every champion in the game, or they wouldn't be reliably able to mindlessly splitpush and riot would have to actually try to balance them around teamfights.
Mages don't need to tankbust by default, but they definitely need to be able to itemize for it, and the itemization should be accessible and effective in all situations. Otherwise tanks that buit 6 Mr and hp items would be able to soak skillshots with their large hit boxes, which wouldn't be fair.
Divers don't all need to tankbust, but they need to be able to itemize into tank shredding, and their kit should be able to 70-0 any given tank from levels 5-12 with a bare minimum of ad. Otherwise, it would be effective strategization to pick a tank with good laning against an aggressive fighter and build armor, keeping them from acchieving their goal of snowballing the lane and taking over the game before your adc and support come online and renders them irrelevant, and they'd only be able to counter your strategy by planning ganks with their mid or jungler or laneswapping with their mid or their botlane or building defensively and reluing on utility or playing a little more strategically instead of mindlessly trading every time their cooldowns are up, and that's an unacceptable expectation.
Tanks need to have largely %hp damage, because otherwise they either do no damage and would have to be actually made genuinely durable, or if they had significant flat damage they could occasionally threaten out of position squishies, which would be ridiculous.
Adcs need to tankbust super well by default, with some of them specializing in Tankbusting in particular without any real tradeoffs, and with the option to focus their itemization on prioritizing Tankbusting, because otherwise heavy tanks don't really die when they're played well, because no other class really deals significant damage to tanks. Tanks would be really overpowered if adcs didn't melt them, nobody else can deal with them at all.
So you see, it's completely essential that the vast majority of champions have a powerful tool, either in-kit or through default itemization, that allows them to invalidate literally the entire point of a tank's kit and itemization. If you couldn't ignore a tank's defences on every champion in the game, what would the world be coming to?
