The problem with what you're saying is the champions you're trying to compare. Don't try and measure a bruiser against a tank, because that's a losing battle.
See, this is how it works:
Tanks > carries
carries > bruisers
bruisers > tanks
and then assassins are just sort of in there swirling around depending on how they've built, how your team has built, and how good they are at what they do
Why do tanks beat carries? Carries rely on being able to keep up a certain level of constant damage, and they try to heal faster than you can deal damage to them. But they still have to be able to kill you before you deal significant damage to them. If a carry and a tank are in a 1v1, and both are equally itemized, the tank actually comes out on top. Why? Because the tank is directly mitigating the damage done by the carry, which is in turn mitigating how much they're healing, while the tank is dealing a constant, if low, source of damage that the carry has very little means of mitigating in return. Tanks have means of keeping someone in range, negating their ability to kite effectively, whether it be slows, hooks, knockups, or stuns. Some have silences to prevent the carries from casting damaging or movement abilities. By directly hindering the basic functions of the carry, and equally built tank makes them ineffective overall.
But why do carries then beat bruisers when bruisers are just tanky damage dealers? Bruisers rely on being able to get in close, but they very rarely have ways of keeping close. They have to deal sustained damage, or use guerilla tactics to hit someone, back off, come back in when their skills are up and try to keep going. They're at a midpoint where their defenses are built up just enough to allow them to deal damage for longer than a carry, but their offenses are built to where they can deal more damage than a tank. However they don't do either of these as effectively as the other two, so if equally built a bruiser will usually lack the capacity to effectively hinder a carry (most bruisers lack CC outside of their ults like snows or stuns that keep someone close for a long period of time). As a result, carries have a much easier time of kiting a bruiser than they do a tank, and as the bruiser hits the danger zone where it becomes a gamble to continue fighting they must quickly decide between an all in attempt or simply taking the lost trade and backing away. Their defenses are only meant to hold up for a short time, but a carry can deal a much higher amount of damage if given more than a few moments to do so.
So if bruisers are basically ineffective tanks, why is it that they beat tanks? Because while tanks deal a constant, progressive damage to a carry, the damage that they're doing to a bruiser is being mitigated by the bruiser's defenses, while the bruiser usually has some form of armor penetration or reduction that's made specifically to hurt someone who's big and buff. I.E. their abilities leave you with a bruise. They aren't enough to kill you outright, but when you get hit by it you feel it, even if you're a tank. Consider bruisers as the armor piercing rounds to you being the Howitzer. Yes, you can take a hit or two from them and keep going. But take too many and you're going to start having issues. The bruiser can reduce enough damage from the tank's abilities to stay in combat long enough to put dents in your armor and get through in the end.
The reason that bruisers seem to be stronger than tanks, and sometimes even than carries, is because bruisers have strong early and midgames compared to either tanks or carries. Because they can survive so well they usually have better laning phases, and with only one or two kills they can usually get a very powerful snowball going as they can do more early on with the mid-tier items like a Brutalizer, a Tiamat, or a Warden's Mail. They can get fed very early in a game. But as they go into the late game the failings of a bruiser become apparent as, much like a Lee Sin, you realize that they aren't dealing as much damage as they seemed to be in the beginning, and they aren't reducing as much as they were when they were dealing that slightly higher damage. They
And then the 21/2/19 Diana you forgot about jumps at you from a bush with a Void Scepter and shoves her kama somewhere very uncomfortable.
Or she fails to kill you and has to run away because she didn't hit you with her Q. Such is life for the assassin role.
. This largely stems from the fact that tanks lack a core item as powerful as other classes to multiply their survivability. ADC's have 
and 
to greatly multiply their power. Mages and AP assassins have 
, 
and 
while AD assassins have 
and its build-paths and 
.
, 
, 
and 
as they give multiple defensive stats. All these items are unable to keep up with their offensive counterparts however, and even in the jungle items 
is less cost efficient than 
or 
. The idea that any amount of changes to the jungle can help to bring tanks back into the meta is flawed, tanks aren't not played because the jungle doesn't favour them, they're ignored because their itemisation sucks.
Tanks can't use their god-tier core item ( 
), because simply they are countered too much in stacking health by BoTRK
