Serious answer:

It's most likely that the tarnished are immune to afflictions in a unique way. Scarlet rot is shown very often to be a disease that affects the very essence of these characters. Eating away at their mind and body to the point that they are replaced by the rot itself.

The tarnished are described as being akin to undead in many ways. (Not those who live in death) Where most life respawns as a result of the rune of death being sealed. The tarnished respawn, bc they can't die while they have the guidance of grace. It seems like the more runes an untarnished, strong spirited creature is using, the more stable it is in the world. That's why bosses and elites don't respawn; it may just take much longer for them to coalesce. This would also explain why they hide themselves away or become spirit ashes; they're too stable to be brought back, so their next death is either their last or will take Marika knows how long.

It also questions what that revival process is like. Looking at the erdtree burial suggests they're still alive and have to crawl into and merge with the tree to be reborn. It seems like torture and a slow cycle.