No body, no evidence. I am pretty sure that LEO "knows" who did this...but I doubt that the police were even able to talk to the person, after all, if LEO doesn't put you under arrest, you do not need to even speak with LEO. No evidence, no arrest. Especially in the case of an adult, just because an adult is missing, left everything, never contacted family members, there is still an outside chance she just decided to leave. Unprobable? Yes, but it can't be completely ruled out, unless the body is found. Or evidence of a crime.
It is frustrating. I feel sorry for her family.