most midlaners can have more health than a Full tank sion jungle in the mid-game.
I don't recall the exact math, but health needs resistances to really "oomph" the effective health. Sure, a mage can have the same health as a tank, but without resistances they're going to be somewhat squishy. It also makes them weaker to items like
that happen to be ADC items.
Every class has access to at least one specific type of defensive stat. Mages generally don't have access to building sustain comparable to their AD carry cousins, and therefore get a higher HP pool. Lifesteal does increase your effective health pool as long as you're in a fight, but keeps your total health pool low to keep you weak to burst/assassinations. HIgher HP in mages, but lower defenses, makes you weaker to sustained fights but at least helps mitigate some upfront burst that doesn't scale with your health.
So yes, comparing it to a tank on paper does make it sound OP, but without as much defenses your effective HP isn't as great. Without access to lifesteal (I'm talking in general terms here, champion specific cases with sustain such as Swain and Vlad get complained about on a daily basis), their HP pool gets to be higher than average.