It is a good clutch heal when you can find time as a main tank. But the extra healing is mainly an offtank tool. So scaling off str makes sense.

Also, makes no sense to scale off the tanks health, as it is not a self heal. It is a heal intended to use on someone else and give yourself some in the process.

Lastly, the devs have made it clear through their design, that for real endgame, they don't want tanks to be stacking only vitality as it is simply a boring stat. The idea is take the only the vit you need and all the str you can.