The simple fact is that within U.S. borders alone, there are maybe 300 million animals, each weighing in at probably 150 lbs (75kg) or more. They drive cars visible from space. They flip light switches on and off wherever they go. They clock in at work. They clock out. They walk in front of cameras, and there are cameras dotting every place of business, and in homes and outdoors. They plug things in and then unplug them. They power on machinery. They use credit card terminals. They visit ATMs. They buy things in cash, with serial numbered currency, which passes through more optical scanners than you might guess. And almost everyone speaks English, exclusively.
And then there are the mobile devices, the uniquely identifiable radio beacons we've, almost all of us, have volunteered to be tagged with.
So, VPNs, and proxies, and TOR, as much as they do, are only going to go so far.
Just look into false positives on the no fly list. Sure, it's slightly better than random chance, but odds are if you pick an name from that list they are a non threat. (It's far worse than that you can probably pick several names before finding a real issue.)
Put another way, if you have no terrorists going through, then your false positive matches will always be greater than or equal to your positive matches, and in this case, equal only when they are both zero, since there would never be a real positive.
In a less extreme case, such as the reality we live in, we have many millions of non-terrorists and a few terrorists, and even with three nines of accuracy, it's likely your false positive matches far outweigh your real positive matches.