Hoping revival won't be limited to just warlocks. I think when we get the new sub classes titans and hunters will have a revive ability. If that's the case then the revival from light makes sense. Not defending the lack of obvious missing cut scenes.

IIRC the opening was supposed to be different for each race but was changed, this one is meant for Exo players.

The Ghosts hunt down and revive those who can wield the light. The only supernatural ability your character has is that they can convert dead human souls (light) into weaponized energy. The others are unable to be revived because they are unable to wield the light.

If I remember correctly, the reason Ghosts can revive Guardians is because Guardians have light inside of them, and the Ghost used that to revive them. Take what the Ghost says on a moon mission (I believe it is the second one) when they find the dead Guardian there, and couldn't revive him: "not even the light remains." This shows that the Guardians need light. Now, I realize that this may cause issues (although solving the problem of respawning being canon) with Guardians be having to worry about anything but pain. But how did that Guardian not have light remaining? Think of the light like a soul. The reason (or, at least I think this is it) there are Darkness Zones and why Hive are so dangerous is probably because they destroy your light, thus your Ghost cannot revive you. Just my thought, thanks for reading.

I read a grimoire about a ghost on Mars that couldn't revive any guardians because the environment had already taken everything but bones

There is no evidence to support an unlimited number of Ghosts. Ghosts according to the Speaker have a reputation of being picky for a reason. Clearly there aren't enough of them to save everyone and not everyone is capable of being a Guardian and protecting the Ghost as Ghosts can be killed and the Guardian goes with them.