I can understand why SCHs would want this since it is a QoL.

From a programming / design perspective, I get the impression that it was less time consuming / frustrating to just make a new skill that could only be used out of combat versus trying to find a way to give Stoneskin the ability to AoE while out of combat. Is it laziness? Perhaps. Given S-E's coding before with XI and now XIV and how many issues they have trying to deviate from the existing programing...

*Glares indignantly at the inability to queue DF with the Chocobo Companion out*

...what we're asking for is probably too resource intensive for a QoL change such as this.

With that being said, do you really need to have Stoneskin for SCH to AoE in any dungeon / content where you WON'T have a WHM? Not really =D In fact, chances are you'll apply Stoneskin at the beginning of the dungeon then just kinda reapply it on the tank after every pull. Otherwise, it'll be a convenience after select trash pulls (Sastasha HM Lamias) or most bosses.