I don't know what it is, but I cannot keep my clothes clean when I'm in the field. Everyone else on the field crew will leave the site looking like they've spent a day working hard, sure. But I leave the site looking like Pigpen. So why the mess?
1. As a geologist, I have to really get into the soil. I'm logging cores, or venturing close to the drilling operations to collect cuttings. So I have more contact with dirt, or with drill rigs spewing stuff in general.
2. I finally have all my field clothing that's sized appropriately and works for me. Everything else I have to wear? Not so much. Traffic vests billow out and catch on things, or just drag down across the table when I'm bent over to log something. Hard hats and safety glasses slide down, and I push them back up with dirty hands or the back of a dirty sleeve.
3. If I'm primarily observing contractors or others, it would be nice to kick back with a camp chair in the shade. But the appearance of laziness doesn't go over well with the general public, clients or other stakeholders who drop in, and the people you're overseeing. Besides, I'm far too twitchy to just hang out. Instead, when I've been on my feet for ages and need a break, I tend to lean against/perch on whatever's handy: a tailgate, a fender, a pile of core boxes, etc. And those are invariably dirty.
4. I'm just not that fastidious, and I'm usually in a hurry. So I lean in to yell something at the driller over the noise of the drill rig, and my shoulder brushes against a glop of grease on the rig. Or I round the corner of a truck and snag part of my jacket on it. Or I grab a mud-covered pipe wrench that the driller can't quite reach, and transfer some of that to my sleeve. Or I'm fighting through undergrowth to locate something, and I get a colony of burrs stuck to...everything.
This is why I need lots of pants and field shirts - I run through them faster than anyone else I know.