Hank Green had a quick video on this recently. Airports like LAX are laid out in such a way that it can take up to two hours to get from the curb to the gate. Rather than try to offer more targeted advice, passengers are just given a flat 2-3 hour number to ensure the majority of people will get in on time. It's overkill for most airports, but you need to do your due diligence

ETA: The mentioned video