Hand holding has already been discussed.
Chest-beating probably isn’t done because people just don’t know.
As for not bowing in the Creed, I have my own theory as to why many many many people don’t do it:
Bowing is, socially, an awkward behavior, because it averts the eyes. When one bows, generally–but not always–the eyes are also downcast. Also, that is the only time in Mass when the people bow.
If the traditional posture of genuflecting was prescribed always, I am certain that most everybody would do it. First, everybody could see the priest clearly doing it. Second, everybody could see everybody else doing it. The social motivator, a very strong motivator (don’t discount it) that is present from seeing everybody else do it, which itself is possible because the body is still erect, is extremely important for these kinds of things. Third, people already genuflect in church two other times usually: once before and once after Mass. It is an act that everybody knows how to do, even if many don’t do it very well!
Finally, bowing is a very liberally interpretable thing. What’s a bow? A profound bow? Who defines profound bow? How far do you go down? At least with genuflecting you know exactly what it is supposed to look like, even if some don’t do it all the way, either because they won’t or can’t. In any case, it is very clear-cut that a proper genuflection is one in which the knee touches the ground.
It is my opinion, very respectfully put, that the rubric of bowing during the Creed simply does not work as a matter of practice. It is done in some parishes by many or most people, but in my experience, in most places few or no people do it at all.
I am quite confident that my assertions would hold up in a proper experiment, and I would love to do one.
