just curious I just recently found out that they have Death:from Age set to false as a caste of their own yet they breed like rabbits (granted my sims have a lengthened pregnancy)

I'm not sure but we know twallan designed these mods around his gameplay style so I suppose he wanted them to be immortal much as they were in sims 2. I'm sure he preferred population control via the personality module for them in which they get slain. Alas easy enough to toggle to your preference.