@SuckingDownDarjeeling The thing is that they may see it, but unless it's by someone who isn't very good at hiding it, they won't se it immediately. They might have suspicions and in that case could suggest meeting the partners individually and suss it out.
BUT a) not all therapists are the same and b) they have to start off by taking each person's word for what's going on as their view of the situation. Imagine going to a therapist who treated you like you were lying. It wouldn't work. If one partner feels ganged up on, or is being told they're abusive when they're not, then that also wouldn't work.
Similarly, a partner can manipulate it to appear like they're being abused, when they're the abuser.
In all therapies the therapist only has the client's version of events to work with and that's actually all that's necessary usually. In couples therapy that becomes a whole lot more complicated.
There are some versions of relationship therapy where there are two therapists in the room. One is doing the therapy and the other is essentially watching the three interact, but also observing the partner who isn't speaking, while the other is. Things that the first therapist will miss because they are only one person. It's not that common (and is expensive!!), but that seems to me a better way to do it.
Basically if someone is able to come across reasonable and lively to the outside world, but abuse their partner, then they're going to do a very convincing job of continuing that already-perfected facade in front of someone they believe is judging them. The abused party stands little chance of coming out unharmed.
At the end of the day, therapists are people, not miracle workers or mind readers. If they spot abuse they'll act on that, but if someone is lying consistently and well, there's no immediate reason for them to know that..and in the time it takes to figure it out, the victim has been further abused and bullied.