Creatures of habit. They've (most of them anyway) likely been marking the same way since back in the junior game before hitting the big-time. Look at Tiger and his blacked-out Ping putter grip he used on his Scotty Cameron's, just because it was what he was comfortable with from playing if for so long.
Then there are those who do exactly what you suggest. Sergio has his personal "logo" stamped on his, and there are others as well. I play personalized balls i.r.l. as well, but still like to put 3 red or blue dots on both sides right beside the numbers....several times per round I usually find the way it lands the personalization is covered, especially in the rough. This way I've got 3 sides covered and one of those will usually be clearly visible.