To play devil's advocate here, but DRKs invuln isnt quite like the others.
It doesn't change the fact that it is still DRK's invuln skill, though. To which I ask again: why should theirs be automatic and the others not?

I want to clarify that I'm not saying the ability doesn't have its problems; people have already gone over them well enough in this thread so I won't repeat them. It's just this approach of "make LD automatic" isn't going to do anything in terms of solving those problems.

Technically speaking, its the longest invuln in the game at 20 seconds max,
No, it's not. People love to keep saying this, but the only time the ability is doing ANYTHING for you is when (if) Walking Dead triggers, which lasts for 10 seconds, not 20, and even then you're only getting that 10 if you get the last bit of healing needed to remove WD at the very last moment (which can pretty much only be consistently done with Benediction from an attentive WHM). Prior to the moment WD is triggered, Living Dead is doing absolutely nothing and you are still taking damage as you normally would, damage that would still need to be healed eventually even if WD did trigger.

Assuming you are using Living Dead in anticipation of an expected fatal hit, there is effectively no difference between using it immediately prior to that hit, or using it 10 seconds prior to that hit. You aren't getting 20 seconds of invulnerability by doing the latter, because until that hit occurs, your situation hasn't changed at all.

It's a similar sort of deal with Holmgang; it's "technically speaking" 8s duration, but again until (if) you take a fatal hit, the ability isn't actually doing anything for you, so the effective duration is generally less than 8s -- unless you can consistently pop it immediately before an expected fatal hit every time.