I know and know of a lot of writers who treat their writing as a professional career, but I can't come up with a single person who I've heard say it's a categorically boring career. A difficult career, yes. An often frustrating career, oh yeah. Not boring, though.
I've taught a lot of would-be writers over 15 years of teaching writing. I think I've finally figured out the biggest single thing that holds most of them back: It's not treating their education/learning of craft like career prep. Instead, they keep it in perpetual hobby mode. This isn't so much about what they do as about how they think. Writing books that publishers actually want to buy, doing what needs to be done to make them successful (to the extent you can affect this), and writing your following work on deadlines that may seem far too short (even after you've negotiated them) is very much a career and a business, no matter how much love for it we retain (or don't).
Becoming a traditionally published writer, and then staying published, is one of the very hardest, most competitive careers we can attempt, and yet so many would-bes don't take it nearly as seriously as, say, their neighbor takes going for a nursing degree. Does the nursing-student neighbor study every day during a school term? With only rare exceptions, yes.
That doesn't mean we have to write every day, literally. But I agree with the poster who said the advice probably got started as a concrete way of telling aspiring writers that they have to write regularly and a lot. Once a day slips by, it's easy to lose another, and then another. Once a writer takes on and owns the career-development *mindset*, though, they can do what works for them and know their writing schedule is okay.