One is surely the workload of
the social workers. They are faced with more
"urgent" cases, such as extreme sexual or
physical abuse. And I recall once a social worker
telling me in Australia that they don't even try to
prosecute emotional abuse cases since there is such
little chance of proving one.
Another might be because the
authorities themselves are usually parents, so they
don't want to blame parents for anything unless it is
very obvious abuse of the kind they don't commit
themselves. Another is that it is just easier to talk
to teens because they are much more defenseless, less
likely to criticize the psychologist or social
worker, less likely to stand up for themselves etc.
Another big reason may be
because the parents often have the power to stop
seeing the psychologist or social worker or to stop
sending their teen. So in short, the parents have
more power than the teens. And it is easier to attack
the weak. And if we don't want to use the word attack
we could say it is easier to mold, shape, influence,
convince, manipulate etc. a weaker person.
Another reason is that people
typically put more priority on helping children than
they do on helping teens. I would say that teens are
almost forgotten unless they have attempted to kill
themselves and someone has found out about it.