Just because there are fans
Just because there are fans doesn't mean there is adequate air flow. One part of the computer might be perfectly fine, even cooler than normal, but another area would show up quite starkly if you were to look at the computer using a thermal imaging camera.
You shouldn't rely solely on what sensor readings say. The problem with air flow is that it's basically a kind of fluid dynamics, and virtually any engineer will probably roll their eyes and groan as they recollect taking a class on that topic. It is not an easy subject to wrap your head around. So your CPU may be running nice and cool, but there's a big heat buildup by the HDDs because there's no room for any air to move, and that is causing some of the problems.
Plus, just like pretty much every other technical profession in the world, there is a wide range of aptitudes of people in IT. Not every auto mechanic is going to be equally good, and neither is every IT person. One just needs to go to the service center at the likes of Fry's or Best Buy to experience this first hand. You might get someone who is way too good to be working a job like that, or you might get someone who has no business being within 10ft of a screwdriver, let alone working on someone's computer.