It's an infectious disease...
It's shared by Zaphod Beeblebrox, Dr Adams from BBC1's 'Holby City' and every single Humphrey Bogart character that ever existed...
But how do they do it?
How is it physically possible for a man (or woman in severe cases of the disorder) to maintain their chin in the transitional state from clean-shaven to bearded? Surely the stubble must eventually evolve into a fully-fledged beard? Surely no-one can develop a 5 o'clock shadow in the morning? But they still do it.
They're following you...
Those individuals who without any sign of personal effort maintain stubble throughout their lives.
There is a possibility that somewhere out there exists a lifeform intent on destruction. It is possible that, in an effort to attract all Earth women, they made the mistake of altering their genetic code to form stubble all the time. It is possible that a lifeform that intelligent could make that stupid a mistake.
But it's not very likely
2. Underground Razors
Somewhere out there might exist a company who makes the 'Stubble Razor'TM. Somewhere out there a 'friend of a friend of a friend' approaches people in pubs and says 'Psst, look at this buddy'. Somewhere out there this company may be silly enough not to go into mass-marketing.
But that's not very likely either.
The most likely explanation is as follows...
1. Take a common urban phenomenon, the joke shop.
2. Make sure it does a really poor trade.
3. Ascertain that the owner is bright enough to realise that many of the more 'macho' clientele attempt to appear 'rough and ready'.
4. Pull strings so that his son can have a degree in chemistry.
5. Watch them create and market a new design, 'The Stubble Mask'.TM
6. Let chaos commence.
To quote a contemporary author (on an entirely different subject):
'These three explanations all have two things in common. They explain then known facts, and they are completely and utterly wrong.'