First, understand that the "field office" is the FBI field office. Not local LE
. Second, and again:
The FBI was given jurisdiction under the Lindbergh Law in 1932 to immediately investigate any reported mysterious disappearance or kidnapping involving a child of tender ageusually 12 or younger.
However, the FBI goes one step further, as any child missing under the age of 18 the FBI can become involved as an assisting agency to the local police department. There does not have to be a ransom demand, and the child does not have to cross state lines or be missing for 24 hours.
Research indicates that the quicker the reporting of the mysterious disappearance or abduction, the more likely the successful outcome in returning the child unharmed.
Emphasis by me.
There does NOT have to be evidence of a federal crime for the FBI to take jurisdiction. In other words, they can take over a missing kid case whenever they want.
But I agree with you that they don't unless there is clear evidence of a federal crime or serious problems with local LE. Without that, they just assist.