To be honest, I think a lot of the people that frequent this forum are younger. For a lot of them, this is their first new car. They haven't had the experience or the background of knowing when you can walk into a dealer and stand your ground on an issue that is clearly manufacturer defect even if there are mods on a car. Most of them also do not have the experience of having owned a car for 100,000 miles or longer in their name to know that warranties are traditionally a relatively short, insignificant span of a vehicle's overall life.
I also think people are also strapped for cash but continue to mod. They want to go fast, but they don't want to assume the risk of having to pay for a repair that's clearly due to heavy use, misuse/abuse, etc, so they tread very lightly around the terms of the warranty, which, quite frankly, are intimidating.
And then there is the issue of leased vehicles. There are people on here that are modding leased vehicles. I'd assume that having a warranty voided on a leased vehicle would have implications upon turning the vehicle in, or trying to sell it if it is still within the warranty period.