Well that's exactly part of the problem. AST's core healing is a clone of WHM's spells and SE has said they're not going to change this multiple times. So this leaves essentially 2 possible states:
1. AST is a weaker healer with raid utility, but is underutilized because of the weaker heals.
2. AST is an equivalent or stronger healer but also has raid utility, so no one wants to take WHM when it's simply worse. (as in the latter parts of HW and SB)

SCH is much more complex to unravel. For one you're kinda wrong, SCH doesn't have the same GCD spellset like AST and WHM do. Even in your own example, Indom - while very good - is an oGCD cooldown rather than a GCD spell and that's rather significant. And of course we haven't even touched the pet yet, which is what most people would say is most responsible for SCH being unbalanced.

I will say that scholar feels more like an expac job than AST does, despite it being part of the ARR launch lineup. On the one hand SCH has the most egregious balance issues, on the other hand, if they made the SCH fairy into a healer-bahamut kind of mechanic or otherwise downplayed its role, I will be kinda sad about that too. But it's not like SE is willing to explore the many healer mechanics not yet seen in FF14, so here we are...