Actually, Tanks often also fall into having a problem.
They gear up and take less damage... But face tougher enemies that hit harder and thus they are at a net neutral. Outside making previous encounters less interesting.
Also, there's that thing where devs just give Tanks more HP. No defences, just more HP. New gear? Only stat on it, Stamina. GG. This I assume is to make Healers feel more powerful, because their new bigger heals are doing more because the Tank is taking more damage because HP isn't defense, it's only buffer >.>
The other thing that happens is that when Tanks gear up... They gear AWAY from defence and into offense. This puts more onus onto healers needing to heal more (But also can actually increase personal mitigation in certain circumstances)
It's all a bit of a mess that no MMO has actually figured out the solution to, they all just copy the same faulty design that has persisted through the years...
* DPS get more damage as they gear up - This is fine and dandy. This is what the role is for and it is universally useful and will always have a feel good impact when you hit EVERYTHING harder.
* Tanks get... More tankiness? Which often is negated by increasing difficulty of fights as you progress... Or by Healers healing more anyway... Otherwise it's just more damage because damage is universally useful (Especially for holding aggro over the increasingly strong DPS)
* Healers get... More healing? But this can often just simply turn into overhealing. Especially if fights include massive HP swings (I.e. When a WHM uses Benediction on a 1HP Warrior that just soaked a TB with Holmgang... Having more healing doesn't do anything here, because it's already a 100% max HP heal on a literally 1HP target)
Tanks and Healers are less straightforward in their progression... Heck, even if Tanks didn't get tankier then healers are stuck in an odd situation, because if they can heal through encounters at lower gear levels... Then higher gear levels would still end up being a lot of overhealing wouldn't it?
Which is usually why in most MMO's when healers get more gear, it simply translates into them doing more DPS because of spending less time actually healing...
Ideally, a company needs to figure out a system where Tanks and Healers have their own, dedicated, infinitely scaling way of gearing. That doesn't run into the issues of Tank defence vs Healer healing vs Encounter damage all working to make things really convoluted and often causing Tank and/or Healer item level increases to not have the same feel good impact that DPS get with their straightforward damage dealing capacity.