Because dirt on a machine costs money in ways that show up on a P&L. A caked machine sells for less. It hides leaks, so your techs find failures late instead of early. It cannot go back out of a rental yard looking like the last customer abused it. And packed mud, grease and rock in an undercarriage adds weight, holds moisture against steel and accelerates wear on the most expensive components on the machine.
Heavy equipment washing is not cosmetic work on this class of iron. It is inspection prep, resale prep and turnaround prep.