Actually they do, but you need to treat it more like a market and arb everything to guarantee outcomes, it can be a lot of work especially with only really one proper betting exchange. Someone had a blog where they aimed to make a million from it but they did it on horses as there are obviously far more events each day (and they also did football and tennis but certain sports/bookies rules vary, some will pay on a tennis match if the first game, set or 2 sets have been completed).
The way to profit is not to bet the bookies, there are VERY rarely odds in your favour to guarantee an arb, it's to take advantage of free offers for new customers or gain a legitimate advantage over everyone else, someone I know makes a considerable amount running one of the latter