Most busts probably are indeed in VMC but there is a slight problem to which there is no apparent solution:
The CAS boundaries are not drawn on the ground below. One has to work out where one is, using a process called navigation. That is where people go wrong.
No use talking about poor airmanship.
It's a bit like my business; electronic manufacturing. You make 10,000 circuit boards. 1% are duff due to defects which are visually detectable (with a microscope). It doesn't matter whether you pay an inspector 10k, 30k, 100k, 200k (pounds p.a.) - he/she will never spot more than a proportion of them. A Chinese one will spot more of them than a Brit one (despite getting 1/10 of the dosh) but the only way to address the issue is to modify the process so that the issue is less likely to arise in the first place.
Human errors cannot be prevented. All one can do is modify the process so they are less likely. All the talk about "airmanship" is completely useless bull.