They only have to last 1.5 years under intended use, which is why manuals go into some lengths to tell you how you're supposed to use it (don't use the blender when wet, only use it a couple minutes at a time, etc). And if you engineer your product to last 2.5 years under those conditions, only a small fraction will break before the 2 year threshold, and of those only a small fraction of those will lead to actual warranty claims (unless it happens early in the product's life)