Who is "they"?
They as in Google, Amazon, Apple, Facebook, etc.
Those are the companies providing these smart IoT devices to the mainstream. Also Windows, macOS, stock Android, and iOS, all of them come with ads and tracking by default.
You can buy FOSS products and self host. Smarthomes can be private and secure.
I agree with you, that's why I continued typing and mentioning all the OSs you can have in different devices that are open source, so that you have full control of your devices.