There is also a significant difference between those games, and the Sims. That is the amount of DLC and how long after the initial game the DLC is created for it. They may not end up announcing Sims 5 until they are ready to stop creating new packs as they believe they will lose profits from it.
Realistically, I don't know that it would necessarily be a terrible strategy for them to create the game and make it fully ready, then randomly drop it one day and do marketing from there. Given it is a game that is expected to have a longer tail, and if they are confident with the game, realistically is there a reason to have a big bang day announced ahead of time? But I guess I'm not a marketing person so who knows what would be best.
I personally would like them to make 5 the best game they can with good gameplay and decent graphics.