Environmental factors come into play too. Ozone in particular tends to break down rubber, and trace amounts of various solvents and other chemicals can attack it as well. Elevated temperatures increase the speed of any reaction, including the deterioration.

I suspect that the long term stability of the materials was frequently known, but if you are building something with an expected useful life of ~5 years, and a material spec'd to last longer than that costs more, you can guess what will happen. When you are designing something to be mass produced, you usually select the cheapest materials that are deemed good enough that a large majority of the product is projected to last well beyond the warranty period.

All of these drives are FAR beyond their intended lifespan. It isn't that they were designed to fail, but the designers assumed that most of them would be scrapped long before now due to obsolescence and I would wager that to be the case. If you buy a brand new hard drive today, you probably aren't too worried about whether it will still be working in 20 years.