I think a lot of it depends on geographic location and what your 'real career' is. For Herb my understanding is that stringing is his full time thing. Many people that do it full time also do tournaments and/or college teams. The tournaments look good for the company that the people work for in attracting and retaining business. They get to say their stringer has worked X tournament and strung for X pro. It's a marketing thing that makes them more lenient with time off than they otherwise would be. As a school teacher, it does nothing for my district or school to allow me time off to do tournaments from a marketing stand point. It probably reflects poorly that I do it not only because I enjoy it, but also because I need the supplement to my income. Additionally to HGXS point pros pay a good chunk for their stringing per racket but the companies sponsoring the stringing do as well. Many of the companies actually lose money to be the official stringing for an event. Between paying the ATP/WTA to allow them the rights to do the event and have their marketing stuff out front, paying the stringers ($ plus clothing/bags/collectibles) , paying for food, paying for lodging and travel, there's really no way to break even without charging $50+ a racket. Even then it may be impossible.Yeah, sorry.....from my viewpoint, that's about the only way I could make it stringing professionally, as a side gig giving up all my free time. I don't see how folks like RP, Herb, P1, and others make it given the expenses and (no small sacrifice) time away from home.
Stringing is a thankless job. and pretty much monieless.
Edit: I would think Yonex at the AO is a particularly big loser. From my understanding they don't pay for their stringers travel (the person must pay) but they also don't charge for the first 5 frames per player per day during the main draw. That's a lot of lost $ when you consider that many pros don't do more than 5 a match. There are multiple exceptions but why not do 5 if they're free?