I think maybe a lot of the problem comes down to a regrettable but popular trend to define value of almost anything solely in monetary terms. So if someone spends many hours on an activity for which there is no possible hope of financial gain (or even secondary gain, as with the argument that, "Fanfic lets me practice and build skills so I can become a professional later on"), and if that activity isn't one generally recognized as in and of itself worth paying money to do (e.g., paying for a health club membership or to attend a sporting event), then to the "value should have a monetary amount" mindset fandom boils down to spending many hours on something of literally no value.