They’re not separate species. Taxonomy is weird. Definitions of species also don’t work 100% of the time. You can argue than humans and Neanderthals are both the same species because they produced viable offspring, but then you get examples like Coyotes and Dogs, which are taxonomically different species but are capable of producing viable offspring. You also get examples where two species usually don’t make viable offspring, but once in a blue moon a fluke happens and one does appear that can make its own young. Miles are hybrids between horses and donkeys who are sterile. Yet there are times in history where mules have had offspring. Does this mean horses and donkeys are members of the same species?