I think you are overestimating how much of a gain those substats are going to be, especially if you consider the fact the issue wasn't present in ARR when the level cap was 50 and going into say Sastasha.

The main cause is 100% the potency adjustments and job changes. Take this example for Paladin. In ARR the main combo was Fast Blade, Savage Blade, Rage of Halone, which were 150 > 200 > 260 potency respectively. Now, knowing that EW changed damage formulas for melees and by association, potencies, I will list the ShB rotation. Fast Blade > Riot Blade > Rage of Halone, 150 > 230 > 270, already we can see an increase in potency. Now, I don't remember if EW made potencies for melee higher to account for the damage or lower, however, the same combo in EW is 150 > 230 > 330 (obviously, this is before the melee mastery trait at 84 to make it a fair comparison) and you can already tell RoH is much stronger.

Another example is Monk, before, Monk had Howling Fist and Steel Peak to use on cooldown as well as Shoulder Tackle. With those being removed, they increased the potencies on their GCD actions, with the removal of Internal Release, Blood for Blood and Fists of FIre, they would have increased it even more, plus GL is a trait, so you don't even have to worry about that either. Monk has just gotten stronger and stronger in single target, plus, in AoE, since TP isn't a thing anymore, you can AoE everything you need to without limits AND they also got a Raptor form AoE in Four Point Fury, which they never had before.

you can make similar cases for all jobs. Job changes and potency adjustments are the sole contributor for earlier levels being easier since everything is balanced around max level and only loosely balanced for lower level caps.