Its because they arent supposed to.
End management is already ridiculously easy as there are many ways to do it.
1. + end procs - my average build has at least three in every one.
2. Enh values in both end reduction per power, and endurance generation in stamina, superior conditioning, phys perfection... Etc.
3. Inherent stamina. It's already free now.
4. Set bonuses. They aren't designed for 1 set bonus or even many to remove all end issues. just like all the rest of the set bonuses have a cumulative effect. But also like other set bonuses you have to actually plan your build with all aspects in mind-res, def, regen, recovery, end, dmg, procs for it to come together.
I'm not one to shrug away things that make it easier but end management is sufficient as is right now.