It's fun. Look at how popular "Frontier" recreationism is. There are dozens of "Rendezvous" here locally alone, never mind the rest of the nation. I'm aware of some organizations and clubs in the U.K. for maintaining historic "fighting arts." One such is a club which maintains historic rifle infantry unit from circa late 16th Century. I spoke with one of them a few years ago.
While people, including those inside the recreation, joke about the viability of their skills after WWIII nukes, the zombie outbreak, or the rise of magic, the truth is that they (we) do it for any number of reasons such as developing a "link" to our own histories or even just because it is fun. That said, there is occasionally "real work" for these skills, often more than people outside of the recreation might think. Two that come immediately to mind are accademia and entertainment. Very often historians and archeologists look to learn the skills in order to get a better idea of how people lived at the time in question. Knowing how a person had to fight with a stone headed club while wearing skin footwear, and how it impacts movement, injury, what concessions and accommodations are required for the person carrying and using, and even what affects these skills and equipment has on local and broader economics. Did a tribe have to trade for chert to knap? (yes, sometimes) Did Scotland have to import their sword blades from somewhere else? (yes, usually England or France) I remember reading one archeologist who claimed that he could tell which people were the flint-knappers in a village by what repetitive stress injuries their bones show. Not only that but the entertainment industry is being pushed by consumers for ever more "realistic" fight scenes. The fight scenes from Ivanhoe in 1952 or Captain Blood in 1935 are far different from the fight scenes demanded now. The viewing public thinks they want to see "how they really fought" with those swords and weapons. (They don't really want to see because it would be a boring fight) In order to generate more accurate and realistic fight scenes, the entertainment industry turns to experts in the field. When the big-budget movie The Alamo, released 2004, a friend and teacher of mine got a contact from the movie production staff asking him to consult with them on Bowie Knife technique.
So while most of us do these things just for fun, to gain a connection with our own history and ancestors, or just to practice a martial art that is something different from Asian arts, BJJ, or boxing, there can be opportunities for income sometimes.