Why are we giving them high scaling for burst damage when they are not suppose to? Aka, ,Malphite, Jarvan, Cho'Gath, Garen, Nautilus, Mundo, Sejuani, etc.
Because there's little risk in giving them high burst scaling.
Tanks and fighter/tanks lack the range and mobility necessary to survive long enough to do damage, so they need to invest heavily in tank stats to get close enough to do what mages, marksman, and assassins can do for free.
Giving tanks and fighter/tanks solid scaling allows them to get a decent return on damage items for sacrificing there durability.
As for most of the tanks you mentioned:
There are 2 types of tanks in LoL:
Tank/supports: Tanky CC
Fighter/tanks: Tanky DPS
Fighter/tanks are a damage roll just like marksman, mage, and assassin.
Unlike marksman, mages, and assassins you don't get mobility and/or range to survive long enough to deal damage, tanky DPS rely on tank so they can deal DPS.
And just like marksman, mages, and assassins - they can kill rival champions with similar ease when played well.
This is exactly the same problem as you had with assassins, except you gave them more perseverance and survivability.
The problem with assassins isn't damage scaling.
The issue with assassins is that everything about the assassin aesthetic and theme tends to run opposite to counter-play.
Counter-play is the general idea that mechanics you use in multiplayer games should add to the experience when you're not the one using the mechanic. Generally Riot's gone for multiple choices to deal with things.
An assassin wants to kill an opponent before they can react, using the element of surprise.
Which completely obliterates most reflexive and tactical counter-measures one can use against an assassin, limiting the avenues to deal with them almost exclusively to strategy (builds, counter-picks, team-wide movement).
The actual damage assassins do isn't the key problem with them, and with tanky DPS, you can see them coming from a mile away. There's often very little surprise when they assault a target, as most rely on walking up to them.
Due to having to build near full-tank to operate, and being more DPS than burst, most have a bit of a struggle sticking to a target long enough to kill them as well - where you can do lots of things to have them not kill you.
traditional adc build can still shred tanks