I'm surprised no one has proposed the counter-question "Why stop playing MoO2 at all?".
MoO3 is a very different beast to MoO1 and 2. Where MoO1 and 2 are micromanaging various aspects of a growing empire (allocate funds to research and production, command fleets etc), MoO3 is much more of a "roleplay the emperor" kind of game. You CAN micromanage your whole empire and decide what every planet produces and control every fleet battle, but the game design clearly has in mind for you to play just one man bossing over his people. You are the policy maker, you write the laws and directives for your empire and your people (AI) will try and live up to those commands. Instead of setting up every new colony, you have already issued a general order for how different kinds of planets should be colonized. Just like in real life, you often end up looking like an incompetent politician who don't understand the real life situation and plights for the people on the ground.
I don't have much experience with the game but I think people who clamor about the game playing itself and that everything is automated are not being fair. By default it mostly is, but you can turn those features off. I guess the problem is that MoO3 is so complex and detailed that it fails to deliver the right response and entertainment the other two games provided when played this way. MoO1 and 2 are very zoomed out, you make more general decisions and get immediate results which is far more engaging than trying to do the same in MoO3 which would involve browsing through tabs, crossreferencing the manual and just dealing with tedious little details to make a whole that resembles a fast and fun empire building game. Micromanaging MoO3 to play it more like it's predecessors is just too slow and unengaging.
If MoO1 and 2 are fast play boardgames, MoO3 is the grognard simulation monstrosity that just can't seem to engage anyone who doesn't marvel at it's design more than they enjoy the outcome of the game.