I think the issue is, a missing persons case is not yet a crime. I haven't looked for stats, but most often it's either suicide, an accident or someone walking away from their life and not wanting to be found.
And, as police frequently say, it's not a crime to run away/leave your family, or even kill yourself, and it's nobodies business why, especially not the police.
We all have a fundamental right to personal privacy, enshrined in legislation. My spouse, my mother, don't own my personal information, unless I'm proved to be dead .
Privacy legislation means, unless it's potentially evidence in a crime, police aren't allowed to take someone's computer and rummage around for details of their private life. Police can use their powers to monitor financial transactions, online presence and phone use, but exclusively for the purpose of locating the missing person to do a welfare check. Not to figure out the details of their private life.