Accessing that location data isn’t trivial. The data is typically held by various private companies who put up at least token legal resistance to cover themselves from lawsuits.
Intelligence agencies have their own avenue for getting the data, and on paper they’re not allowed to share it with police agencies.
Police agencies typically need to specify the individual in question, or the specific location and time to get a warrant. This is because they’re not supposed to be able to blanket surveil an otherwise private piece of information without having a good reason.
The classic example is not being able to listen to every call on a payphone they know drug dealers use because they’ll listen to people who have not done anything illegal.
Intelligence agencies are an entirely different thing with weird special rules and minimal and strange oversight.

This is all relevant because the government doesn’t actually know who’s allowed to be here or not.
Most people in the country without proper documentation entered legally and then just stayed outside the terms of their entry. The terms can be difficult to verify remotely, which is why you’re not actually here illegally until you go in front of a judge, they deport you, and then you return again.

Finally, there are significant chunks of the country where location tracking via cell tower is imprecise enough to get the country wrong, and a lot of people live there. So any dragnet surveillance setup is going to have to exclude some pretty large population centers to avoid constantly investigating people in Windsor sometimes quickly teleporting into Detroit.