In my personal experience, I never understood I had a problem, at all, until I was tested and interviewed by the psychologists. Only then, did I come to realize I had "severe" deficits in reciprocity. Sure, I notice things now that it was brought to my attention and I am self-conscious about it, but what I am trying to suggest here is that perhaps we are not the best assessors of this and that perhaps we just might have more difficulties with reciprocity than we think we do. Furthermore, I am in the middle of reading the latest biography on Elon Musk and there are quite a few situations were he has difficulties with empathy and social-emotional reciprocity that can really make him appear to be a total jerk. Others were recognizing that Elon is consciously analyzing people and struggling to read others, but in the heat of an emotional moment, he can lash out without any regard for the other person. As I am reading someone else's experiences it really hit home.
So, to your question why is only understanding social cues not enough? First off, I am wondering if we are truly understanding the social cues as well as we think we are and two, most people are emotional creatures. So, imagine a humanoid robot full of AI communication software and visual sensors to recognize all the soft and subtle body language. It's a logical system, but without the emotional component. No matter how well the outer skin and prosthetics were that could be made to make it appear human, a human being would still quickly recognize this lack of emotional reciprocity and could tell they were dealing with an artificial intelligence. Now, obviously, we are human beings, but without some degree of "expertise" and "fluidity" to our emotional reciprocity, others are going to sense something a bit "off" or "odd". The little warning "bells and whistles" are going to off in their amygdalas and they will be hesitant to make a strong social bond with us. To complicate things, if we are not making those emotional connections, it's difficult to make friends and find life partners.
"Why can't I find a girlfriend?" "How can I make friends?" "I am getting a divorce." You've seen these threads on this forum. In many cases, that missing emotional reciprocity and understanding is a contributing factor.