Because you're reducing the etiology entirely to trauma.

First of all I'm not:

We also agree that there can be a certain sensitivity present to developing problems like this. It doesn't necessarily contribute, but can, and I don't doubt there will be cases where the actual "objective" trauma was very limited. This is also why the experience and memory of the client is much more important than the actual facts. It's what they formed in their minds that matters.

Secondly we might have different ideas about what trauma means. 'Trauma' here isn't meant in the sense that the parent hit the child, maltreated them, abused them in some way. It is also traumatic to a child when their specific needs are not met in the way they need to be met for them. So it is 'traumatic' to the child in the sense that it is vulnerable and dependent, and its needs (which might be quite out of the ordinary) are not met.

Like I said somewhere else, if a therapist can help a person, a parent would also have been able to (and probably even more so, as it certainly doesn't become easier with age).

And when someone astutely points out that people without trauma histories can have BPD, your response is to insinuate that these people are wrong annoy not having trauma, e.g. not remembering it.

I didn't insinuate that, that's what you are making of it. I said that trauma isn't always remembered (and isn't always outright abuse). That's simple fact, so someone saying that they don't remember it doesn't mean it didn't occur (and it wouldn't be the first time client's began to change their view on their childhood during treatment, and certainly not due to any indoctrination on my part). Also I'm not talking to a client here, I'm in a public conversation.