You understand "being secure in your person" is not merely "I don't get physically touched", right? If I stalked you every day without ever touching you, you wouldn't feel so secure in your person, would you? If I was a police officer, you still wouldn't feel so secure, would you? If you knew I was doing this remotely instead of in-person, by monitoring you over video cameras across the city and tracking all your moves with your own GPS devices, you surely wouldn't feel so secure, would you?

People (maybe not you, but most humans) feel threatened when all their moves are being tracked. There's an implicit threat of physical harm even if it hasn't occurred thus far. Not to mention there's also the risk of a bad actor (read: including law enforcement insider) stealing your tracking data that was supposedly only ever being used for good. It's a real threat to your security, and you have a right to be secure. If another person is going to threaten a free person's security, they sure as hell need both the legal authority and reasonable suspicion of a crime. That is the amendment.

Where to draw the line for "reasonable" here can vary somewhat, but I think most people would agree that if you have 3 people all in close proximity to a crime, you could justify having reasonable suspicion of each individual of being involved. If you have a hundred people walking in a half-mile radius, you clearly don't. Idk where the line exactly is, and circumstances can affect things, but somewhere between those seems like a reasonable place to start.