Privacy

Using a simple flip phone with no app ecosystem would fix this, right?

Probably not.

The cell phone network will need to know where every subscribed phone is, in order to be able to route simple phone calls and text messages. In the old days, that might have meant very broad ideas of which areas were covered by which towers (often divided into 3 slices of 120° each from a single physical tower), and maybe some timing data to understand how much to offset the signal timing to make up for the speed of light.

But each generation of cell phone technology has been about adding more capacity into each wireless frequency, and the towers have specialized tricks for transmitting and receiving on the same channel at the same time with different devices, by getting more precise about transmitting or listening in very specific directions and distances, with spatial signatures that distinguish between devices. It's all very cool and mathematically beyond my understanding, but the result is that the towers have much more precise location data about the actual handsets.

So the cell phone companies have your location data. The question becomes whether they sell them to location aggregators who resell the data on the open market, and whether some of the buyers of that data are law enforcement agencies without a warrant.