Why won’t you? Perhaps because you shouldn’t. You should be on to making new work and allowing the mental energy and time you have to be directed towards the present, not the past. That seems like the most pertinent reason.
Perhaps you won’t photograph that work because it is a hassle to pull it from its frame (and, presumably, reframe it). Perhaps it has already sold. Perhaps it was destroyed in shipping.
At this point you may be asking why you should even care about photographing every work before it leaves your studio. In my experience the reasons are far more practical than inspirational (although examples from both will be provided).
On the practical side of things, you’ll want the image so as to include it in your applications for grants and residencies. You will want the image to add to your website. While these arguments are lacking in glamour, their logic is irrefutable.
On a more inspirational note: you may want to reference the image in a future work. You will be working on this new work and realize that the older work (according to your faulty memory, which is the only record you have in this scenario) actually began some trope that is only now being made manifest as such in your newer work. The intrinsic value of that undocumented work has now just skyrocketed, and allowing it to escape your hands without a photographic record seems like the most grievous error you’ve ever made.
In short, do not allow your work to be previewed by collectors before photographing it. Do not let it be framed before photographing it. Do not let a single work you create reside only in your memory because your inability to access a reproduction will haunt you for years to come. Those works that get away will climb effortlessly to the heavens where they will reside as the finest works of your hand perhaps based solely on the fact that you have no way to refute their lofty claims without a digital photo or slide.
While you’re at it, I’ve also found it exceedingly helpful to record the following information about every work you photograph:
TITLE, YEAR OF COMPLETION
DIMENSIONS, FULL SALE PRICE,
More on the specifics of each of the above categories to come soon. . .