When you have a few bunches of products, you will have a bigger marketplace. That's what casinos are focusing on. All business wants to expand their business & there's nothing wrong with running a casino & sportsbook together.
Although there are thousands of casinos, few of them want to be unique by targeting a unique group of gamblers. If the site is focused on Dice games; dice is their main product & they may add a sportsbook as their secondary product. Simple as that LOL & it's obviously profitable. On top, gamblers don't play certain types of games. Some gamblers play slot games & place bets on sportsbook as well. It's an advantage to have both casino games & sportsbook for them.