For the same reason why you are allowed to fish with a single fishing rod in some places, but can't just drop a giant net or trawl the whole river: Scale matter

For the same reason why you are allowed to fish with a single fishing rod in some places, but can't just drop a giant net or trawl the whole river: Scale matters, and regulating the tools is often an easier and more effective solution than trying to regulate scale directly. In particular with data, the processing method makes a qualitative difference, between individual disconnected people each knowing one small snippet of your life, and a megacorp having an overview over whatever you're doing at every moment. By de facto limiting it to "one brain", the panopticon is prevented. Your rights end where they start impacting other people's rights, and drawing those lines is what we have governments for. Even if you were to use just your memory, if you were tracking what someone is doing in such detail as ad companies do, you'd likely a) not be able to do that to more than a few people b) receive restraining orders for stalking.