I had wondered that myself and have had this article saved for a while. It provides some insight.
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Not all individuals indicated as missing are victims of kidnapping, murder, or some other criminal act; however, each account represents the concern of someone reporting a person who disappeared.
Disappearing Voluntarily
A variety of reasons—mental illness, depression, substance abuse, credit problems, abusive relationships, or marital discord—exist for why people voluntarily disappear. Due to the high number of missing person reports assigned to criminal investigators and the vast quantity of cases resolving themselves, investigators tend to “wait and see” or prioritize a case lower than an identified crime. With enough reprioritization, an investigation easily loses momentum and becomes part of the load of other missing person cases. Often, no one submits requests for electronic data or captures timely eyewitness interviews and victimology. Consequently, timelines become vague, and memories fade.
Becoming a voluntary missing person does not constitute a crime. Any adult can walk away and decide to ignore family, friends, associates, and employers. Because this type of behavior lacks criminality, law enforcement officers experience limitations regarding how they proceed.
Investigators sometimes receive inadequate information in the beginning of a missing person investigation. If people portray the victim as routinely running away, being reckless, or acting irresponsibly, others may express less concern and possibly not even file a formal report. Investigators could treat the case as a reported event, rather than a potential criminal act.
However, when facts and circumstances indicate a strong possibility of foul play or the disappearance occurs due to criminal action, investigators should consider the missing person case as a potential homicide.
No-Body Homicide Cases: A Practical Approach | FBI: Law Enforcement Bulletin