I get up in the morning. I take a shower. I wash my neck, among other things. I put on a white dress shirt with a tie. I get home at the end of the day and the collar is dirty, just like in the Wisk commercials.
Where does that dirt come from? I started out with a clean neck. I work in an office, not a construction site.
It’s probably body oils.
I hate that stuff too! Is there a good way to prevent that?
Sailboat
ROFL 
And on that grisly note, the key to getting out blood stains is to use cold water; hot water will set the blood in permanently. 
“Apply directly to the…”
The next time I have to wear a suit and tie, I’m placing the tie on the inside of my collar! 
Which is, incidentally, how they were originally worn.
JRB
At some point in collar-and-tie history, collars were stiff, detachable things, and were easier to wash.
But you work in a city or town, right? With cars and industry and all? Some of the grit in the air gets caught in the sweat and sebum on your neck, and gets picked up by the material of the collar as it rubs up and down the skin of your neck. Similarly for the ring around your cuffs.