The same reason they're not allowed to choose a phone with a walled-garden ecosystem: A few bureaucrats in Brussels don't like it.

Because basically everyone giving up all privacy for a modicum of convenience makes the world a worse place. It's a seatbelt situation; people consistently make the wrong decision, so the option is removed. In the abstract it's distasteful to remove autonomy like that, but on occasion we need to make collective calculated decisions like this.

Same reason you can't sell your organs or sell your physical freedom / time in prison (falsely admit guilt because someone paid you to do so in order for them to avoid prison) or even end your own life. The country you live has citizens who have banded together to created laws and regulations that say these various freedom-y things, if engaged in at scale by people who might have individual reason themselves to do it, are considered harmful to society and so prohibited.

Although the decision sounds so simple the underlying principle cannot be accepted. Privacy is an inalienable human right, just like being a free person. We, as a modern society, decided that some things are illegal regardless of my much both parties agree to do so. One cannot own another person, or work without compensation, or sell their organs to evade prison time. What EU is saying is that such a transaction is illegal. And just like the slavery, numerous companies are doing everything in their power to keep it.