Engines become dirty due to modern day stop-and-go traffic conditions, as well as imposed speed limits, fuel quality, new anti-pollution requirements, and driving at low speeds to save on gas.
These symptoms manifest in the form of saturated EGR valves, clogged DPF filters, and dirty turbochargers.
In the past, it was possible for drivers to clean their engines through normal highway use. High speeds equal hot temperatures, which enables self-cleaning engines. However, this is no longer possible given modern traffic conditions.