These days? Doesn't that mean these feats aren't in either PH1 or whatever is core for Essentials?
They are in the core for essentials actually, Superior Will is the improved will maths feat. For a cheap price of 15 wis or charisma, you can make a character that is very resistant to stun effects.
Not to mention all the other feats, paths and epic destinies that make you highly resistant (or even immune in some EDs cases) to being stunned. Stunned frankly is not as useful for a monster at paragon/epic as simply being able to attack more and inflict damage. Damage kills PCs, stunning them merely annoys them and doesn't work a good chunk of the time anyway.
You might be making a lot of assumptions about what sort of feats characters take.
Epic will is +4 to Will Defense and a saving throw at the beginning of your turn against debilitating status effects.
This is not an assumption. If you have the Will,
you take that feat in a good chunk of cases.
Plenty of people? Sometimes players want to stick with class-based feats or those that make sense for their character.
When you're chain dominated constantly because your will defense sucks and spend more time hitting your allies than you do the enemy, feats like Epic Will become more than somewhat required.
Not that there aren't a lot of other ways of completely negating stun/dominate now for
many characters. Direct damage is far better for solos and if they waste a round doing no damage, it just leads to extending a fight: Not making it exciting.
This is finally what Wizards have figured out. I personally haven't used a solo with an AoE stun attack now in a few months. I use single target stun/dominate effects though, but they come with damage or immediate attacks (or in the case of many no-damage dominates, are minor actions). The worst status effect I usually impose is an AoE daze + damage. I just find the game is better off when creatures and PCs can actually act.
An endless war where everyone is trying to stack stuns/dazes onto one another, without actually killing anything is boring and leads to 90% of the "Epic level is a grind problem". Remove the endless status effect metagame and suddenly you have a fun, exciting epic level that is about killing things faster than they kill you.
Quite frankly, nerfing the crap out of stun/dominate/daze through monster powers (like instinctive action and action recovery) and by giving PCs items, feats, EDs and PPs to deal with it has been the best decision by wizards. Epic and even paragon tier play far better now than they did previously. Bear in mind until monsters like solos started to get ways of genuinely just canceling/dealing with stun/daze/dominate high level 4E essentially was about chaining until end of next turn stun/dominate/daze effects on solos until dead. You had long grindfest fights where the creature did nothing, or alternatively where it repeatedly stunned PCs or similar and did nothing anyway (due to doing pittance for damage).
I do not lament the loss of pointless fight extenders. If anyone asks "What do you do about defenders?" the simple answer is:
Drop them. You have the damage output to do that now.