The FATES in the area mention that Voidsent are attracted to the Ceruleum that's being mined, and the Lambs of Dalamud did once have a base in an abandoned mine in North Thanalan. While nothing is explicitly stated about the state in general the cult was involved in summoning voidsent so that may be the reason for Voidsent still being there, beyond weaker ones falling through naturally occuring rifts.