The majority of doctors don't say i don't know, it's in the culture and as a coping mecanism.
That's the least they could do. They owe it to their patients, who deserve to know the truth. Being a doctor is a highly responsible job, everything they say impacts many people's lives, if someone has a coping mechanism that includes lying just to protect their ego, then how can we call that?
And that's not even the worst part. They don't know, so many of them instead of saying they don't know or suggesting to go to some other specialist, they persuade patients that they are mentally ill, how evil is that? Many patients then take drugs with a lot of side effects and get worse, because this is not the right treatment. All because of some doctor's ego. Why a doctor should be treated differently than a common lier? What they do is simply criminal. They do not only not tell the truth, but they traumatize their patients, laugh them in their faces, get angry when patients ask too many qustions or suggest something, or tell them that it is all in their heads. This is gaslighting technique used by sociopaths.
No excuses like coping mechanism. Imagine that instead of a doctor you have a car engineer who has the same coping mechanism and because of that a car is made who is dangerous and kills people. Would that be ok? The difference between doctors and engineers is that doctors can do it, because no one can prove that they lie. I agree that the whole system is flawed, that doctors may be overworked, but at least they could do is be fair to their patients and tell that they don't know if they really don't know, or ask them to come later when they do their research. And how many doctors actually want to do more research?