The gameplay might be finished but the player skill isn't, since games usually get harder and harder or more complex the closer you reach the end. If I have to play a bit now and end it then wait months to play the next bit and it is harder and/or more complex, it will take a toll on my skill and I will lose time to get as good as I was already at the end of the first part, then repeat until all episodes are released. Imagine playing a full game and play 5 hours of it then stop for months, then come back and play 5 more hours and then stop for a few more months and so on. It would make the game boring and depending on the type of game we would have to relearn how to play again and again. I never buy things in parts, games I think I will like I wait until all DLCs and "parts" to be released as a "Complete Edition", because Episodic stuff or playing now and then only get the full product later really sucks for me.