My hypothesis on this is that marketers who have personal MacBooks but are forced to use Windows computers at work, with mice with notched scroll wheels, find JS-driven smooth scrolling to be superior to the native snapping experience they see at work on many websites. But it wreaks havoc on people who already have computers with native high-resolution trackpads. Alas, the folks at big companies care more about their at-work than at-home experience, and it's been cargo-culted to smaller companies now as well. The conversation "detect if there is indeed a trackpad being used" never even comes up.