It isn't "beyond a doubt," it is "beyond a REASONABLE doubt" which means the jury does not have to find a defendant guilty "beyond the shadow of a doubt," i.e., any doubt at all, but only beyond a reasonable doubt.
I don't think the O.J. trial is a good example of American juris prudence. We all think (know) he was guilty, but we weren't in the jury room.
I have served on juries and in one murder case, we found a defendant "not guilty." We all believed he was guilty, but the state just had not proven its case, creating enough reasonable doubt that we could not convict with a clear conscience.
When you're on a jury and you are literally holding another human being's fate in your hands, you want to be absolutely certain before you find a defendant guilty. I cannot think of anything worse than condemning an innocent man or woman to prison or, in some states, death.
Many, many years ago, Benjamin Franklin said, "It is better that 100 guilty persons should escape than that one innocent person should suffer." This is still true today.