are not going to burst your 4000HP/200MR jungler or your 3500HP/200MR top laner.
That's... kind of the point. The assassin/nuker with mobility is supposed to destroy the adc, who's usually protected during a teamfight. If they could 100-0 burst the tank when they're even, there would be a balancing issue.
In a teamfight, this typically happens:
Tank applies aoe damage and/or cc, forcing the enemy team to either 1.) engage and deal with the threat or 2.) peel and disengage from the fight.
The adc autoattacks the nearest target that's safe, typically the enemy tank and/or frontline using items such as Botrk or LW
The assassin/nuker uses their mobility to destroy the adc
The support protects the adc primarily and the team secondary, applying cc on troublesome targets or sustaining the team
The fifth role does whatever they need to, depending on the team comp. Sometimes doesn't participate in teamfights to secure an objective.