A poor diet could easily cause damage to arterial walls. A lack of EFAs, lack of antioxidants, eating things like trans fat, hydrogenated oils, excessive omega-6s, too much sugar, lack of nutrients, etc.
As to why the body would need what it needs...I honestly have no idea. I'm not God

But I also noted that there could be a problem that results in over-production of cholesterol, as well as the body increasing it in response to a problem. There is just a lot that can go wrong in the body when it is fed a poor diet and is exposed to lots of bad things (smoking, evironmental toxins, etc).
The point is I don't see a purpose to just lower cholesterol. It's like when people get a headache from a lack of water then pop a Tylenol...sure, it gets rid of the symptom, but it does nothing to actually address the real problem (lack of water). This does absolutely nothing for health.
So if you have a problem with lack of nutrients and EFAs in your diet, what good does using a cholesterol lowering drug do? Especially when those drugs have negative side effects. All you do is introduce a drug to the body that gets rid of the symptom, has negative side effects, and leaves the actual cause totally untouched.