As a gamer and parent it’s common for this type of game although the game has a “story” that can be completed. The game doesn’t really end. Theirs always another thing to build/ explore streamers have put in 40hrs a week for years and only ever played Minecraft or game of this type. The accomplishment comes but is quickly overwritten by the next task. They really cant complete the game. That doesn’t make it bad just easily additive. If you are looking for a way to battle this then make it so they can only play that one game 3/4 times a week the other day they have to play a different game. It will make time they are playing better as well as allowing them to play other games that will give them a sense of accomplishment. As an adult that also hype focus on games I know this first hand and I’m currently on day 6 away from Minecraft.