A basic single player game, which has a beginning middle and end, has something that a service based game does not. That thing is the “end” part. An “end” provides a sense of “completion”.
Let’s take a single player game that this rule does not apply to, and use it as an example. That game is Dead Cells. When you beat the game, it just gives you the option to play on the next difficulty. You don’t get the true final level until you complete it on the fifth difficulty. The initial completion comes with no ending, no credits, nothing. This makes the initial completion unfulfilling, and feel more like finishing a level than finishing a game.
Do you seriously need me to keep explaining this concept to you? You really need someone to explain why things with endings have a sense of ending? This is so obnoxious and boring.