Arent they literally created by drowning people in some tar shit, pretty sure it was said explicitly multiple times that they count as undead
That's a ritual to convert a Non-Vida races into a Ghoul.
Ghoul's only use that ritual when their clan are on brink of collapse, otherwise they avoid it.
The concept of this Ritual here is more to do with Rebirth, and not being killed off.
There are other races mentioned in this series which also have similar suicidal type of race conversion method, and they are not related to Undead in anyway.
Normal Ghoul's are born through normal conception, but they have a weak fertility because the body of a female Ghoul sees even the baby growing inside itself as a foreign infection and tries to kill it. So, they have low population, and because of hunting by Adventurers, there numbers fall much more.
As for the reason they are counted as Undead, its primarily because of the false & misinformation that had been spread under the influence of Alda Faction, who sees all of Vida's children as abomination. Many races don't even know the fact that Ghoul's are actually a race of Vida, instead they treat Ghoul's as monsters.
The only reason Ghoul's are related to the Death attribute is because, they were born after the dead body of the Fallen Hero was revived back (without his soul). In that state he was neither dead or alive, so his children, aka., the True Progenitor Vampire & his Ghoul twin got connected with the Death attribute.
Well, like i said, Ghoul's are as much Undead as Vampire's are.
The primary reason to remember the difference is because this series is quite specific on who is what, because of the whole relation with Death. I mean, any being which is related to Death even indirectly fall under it, like the Cemetery Bee's, those bugs are not related to Death in any way except their name, and thus fall under its influence.