There is nothing to keep you from punching a bigger hole. I assume this is so they can pull a DB9 through it with the shroud in place. We used to take the shrouds off but you need a cable that doesn't have the molded connector for that.
Plastic boxes make this operation easier since you can use a $3 hole saw instead of a much more expensive KO punch. I have seen IT managers, with too much time on their hands, cutting holes in boxes with a Dremel tool. I figure those holes cost the company about $50 each.
