Because the economy is the basis for the rest of the simulation. It’s not something you can just refactor in a DLC, you’d have to practically rebuild the game from the ground up around it. Which isn’t technically impossible, but the likelihood of it happening are slim to none. The proof of this is CO’s last game Cities Skylines, and in fact the very existence of CS2. They added snow and snow related services in a DLC, but they couldn’t/wouldn’t implement it into the base game, or even implement it into existing maps. Then they released the industries DLC which overhauled industry mechanics, but when they released DLCs later that added new industries, they couldn’t / wouldn’t integrate them with the same system used in the industries DLC. And those were incremental new features, not total rewrites of how the city’s economy works. Half the reason Cities Skylines 2 exists is so they could actually roll all the changes made over the years by DLC into the actual base game. CS2 is little more than a refactor at this stage.

And the economy isn’t fake in the same way all economies are fake. It’s fake in that it seemingly doesn’t exist. There are numbers and graphs and features in the game’s UI that don’t actually correlate to existing parts of the simulation. At least, that’s the way it appears, given extensive testing by the community. The developers have insisted these mechanics do exist, they’re just broken. We’ll have to wait and see, which is what I’m doing.