Reusing models saves production time which lets employees work on more overall.

Tanks tend to get the most unique model meshes thanks to the fact that 1. All tank classes ever will just pull from the same pool of gear until the end of time and 2. Square Enix want it to be easy to visually show that Tanks are the heavy metal wearing punching bags that the role expects from them. DRG for the most part was usually getting their own sets of gear because DRG is the only current DPS in the game that is using "Scale" tier armor, which comes with higher physical defense than your average DPS. That doesn't change the fact that they have been recently reusing and slightly tweaking tank gear whenever it seems convenient.

Unfortunately for healers, they are still in the same gear category as casters, which means it's very easy and very convenient to just reskin some robes and call it a day.

Like I can guess it kinda sucks that tanks are always getting their own special snowflake gear, but reusing models is a design concession, and an unfortunate fact of life in terms of MMO development. And I for one would much rather have development be put into more actual gameplay than just to further advocate the perfect waifu-simulator.