Because it's not as simple as that - You have to at least complete the first introductory bits. After that, certain events in a characters progression are tied to stats, which require grinding and on that note - when it comes to unlocking the gym, also some sidetracking. Others require money, which you gotta earn somehow. Just starting up the game and completing a character arc from start to finish is not gonna be possible if you don't do some sidetracking. Which is why people want save compatibility to just experience that from start to finish without doing side activities, which I understand.
On the other hand, the extremely long updates have the added benefit of me wanting to experience the game from start to finish anyways, so I don't mind it that much anyways.