Think of it like trying to get a "crack" for 37s Basecamp or similar.
You might be able to eventually do a good enough job that makes your server feel quite close to the official server but then every time the game updates you either have to stick with the old way or do all of your analysis again. So you end up with a divergence between the real game and the pirate game.
It is certainly much more time consuming than figuring out how to set the "isGameValid" in securom to 1 and breaking every securom game out there. In this case you have to individually reverse engineer every single game out there, so at some point you hit the limit of the rescources that game crackers will spend on cracking. Essentially the problem with traditional DRM is that attack is easier than defence, this inverts that relationship.
Some games, WOW for example have people spending significant resources on reverse engineering them (although for different reasons) because there is a financial incentive to do it (you can make money selling rare items). There probably isn't much money in cracking simcity.
But game runs just fine for twenty minutes after disconnect (at least it simulates one city fine). You just may lose your progress because game saves "in the cloud".
They're not stupid, they know it's getting cracked. It's just a play for more time.