Many online casinos in the early days developed their own casino software. The trend shifted a long way back in favor of external providers that could focus on development only. It also gave smaller development firm the chance to niche themselves and innovate and still have the opportunity to supply one or more casinos if the product were successful with customers. A network like this alows everyone to do what they are best at.
Software belongs to the core of a user experience for online casinos. Software providers are plenty and most casinos usually buy services from external software providers that are experts in their own field. This also makes for economy of scale since the cost of developing new games and making sure that old games are updated fall on several gambling companies instead of just one.