What is blatantly obvious to a person isn't exactly the same to a machine. We see and are able to make logical connections that a computer is not programed to do.
Looking at wall hacking. Should you ban everyone who wall bangs people more than once a game? How many times does it take before they should be banned? What if the other team just is that bad that they make obvious noise and get wall banged every round?
When you watch someone wall banging you can see their cursor trace them a lot of times. You could do the math and track where a user is pointing and exactly what they are pointing at but how far do you track it. Through one wall? all walls? A fair number of calculations occur to just figure out something that most the time isn't happening. Then what is your fault tolerance? Do you ban someone who has traced someones head for 1, 3, 5 seconds? In game it often (by the hackers who are at least a little bit smart) takes less than 3 seconds from start of trace to killing you or moving on. However, it is possible while running through a corridor that you are pointing at someone. Should you be banned because you were moving down a hall and an enemy was stationary behind a wall but you had no clue of that? You didn't shoot him though. Should that be the trigger? How long you traced + shooting? Is it not possible to get false positives? Should you ban innocents just to ban a few bad people? A smart player may know an angle and end up tracing someone but not be wall hacking.
Then lets talk about this saying that we say 3 seconds of tracing plus killing the target. Ok, now what is your tolerance for tracing? I learn that if I trace more than 3 seconds I am going to get banned so I just get slightly in front of the target for the place they will be and then fire as soon as they are there. Making sure that my time spent on them is less than 1 second. How would you prove they are wall banging in the machine? They just got around your algorithm.
It's not as easy with a computer to prove someone is hacking compared to with a human. VAC bans are handed out after gathering enough evidence that the computer is sure you are a hacker. Have there been false positives? I don't honestly know. However, I know that most the people it catches actually have a history of hacking and while they may not get caught today that doesn't mean VAC isn't watching them.