Don't even watch heavyweights, it'll ruin you :yep Except maybe Adamek, he's got a good style for amateurs to watch. But to answer your question, it's got a lot to do with the fact that they don't score blow-by-blow in the pros like they do in the amateurs. A pro fighter can take some shots and not care, but in the amateurs a clean punch is a point regardless.
They tend to have excellent knowledge of range. While they might look like theyve always got their hand down most actually put them up when in range.
In Briggs' case it was fatigue for sure. What I like to do when I get too tired to keep my hands high in sparring is from time to time bend my knees a little more and make myslef a smaller target....It's easier to keep your hands by your face when your knees are bent more.