Because it’s still more expensive to install the heated seats for everyone then to not install them at all. And most people don’t need them. So if it was a base feature then the car would be more expensive for everyone.

So now they let the people that bought the upgrade pay for everyone’s heated seats - which is less expensive then it would be if heated seats were only installed in their cars.

I’m not saying that it’s the right thing to do, but it does make the car cheaper for everyone.