Why don't astro cards stack?

[Seraphor]

Ironically, regens do stack, all of them.
WHM and AST, two WHMs, or two ASTs, you can stack all their regens.

Edit due to daily post limit:

Two ASTs cannot stack their regens or their shields.

WHM + WHM = stack
WHM + AST = stack
Di AST + Di AST = no stack

No they do.
Shields don’t stack, but all regens do stack. Play dual Diurnal AST and you’ll see the two sets of Helios and two Benefics on your tank.

It's a shield thing, not a dual AST thing.