Why? Simply put it is the cheapest solution and homeowners think it’s the most cost effective. This was true back in the good old days when a 20 year asphalt shingle would last 20 years plus.
What Happened? Asbestos was a key ingredient in those shingles and the main reason for their longevity because of its tensile strength, and its resistance to heat and chemical damage. Asbestos is used for its resistance to heat or fire, the fibers are often mixed or woven into fabric or mats (i.e., asphalt-asbestos felt, shingles and surface coatings).
Asbestos in 1989 was outlawed but not banned in Canada. Asbestos kills thousands of people every year and causes cancer, yet Canada still mines and exports asbestos but not for use in Canada.
Post 1989: Ever since they outlawed asbestos in building products and roofing shingles they typically last 8 to 12 years. Now typically made of paper or fiberglass impregnated with oil and coated with fine stone, they begin to break down as soon as they are exposed to the elements, but the manufacturers still call them 20, 25, 30 and 40 year shingles.
How do manufacturers get away with this? In the FINE PRINT the so-called 20, 25, 30 and 40 year shingles come with a 3 to 5 year warranty and a pro-rated declining warranty that has a bunch of loopholes. The warranty is based on shingle weight and thickness, hypothetically the heavier and thicker the shingle the longer it will take to deteriorate.