Answering for myself, it is an issue of time and accuracy. Now I'm assuming that kit makers as well as the myriad of companies that put out AM decal sets have invested many hours into research and post printing quality into any given subject ( you can actually tell when they haven't done this). More than just an afternoon I'm willing to invest in such a project. I'm also assuming the research team has individuals, or at least one person going out in the field and taking notes and whatever of the subject, whether it be at a museum, an airforce base, etc. and is not just sitting at his laptop googling images.
Copying an existing design to the computer is a great idea, but the final image will only look as good as the original. Using photographs - maybe for the modern subjects, but what if one only builds WW2 where most photos are b/w.
I have printed decals on clear film in the past. I've found that even laying them down on a light surface the colours are just not as vibrant and end up placing an identical decal on top. Still though, if the decal image requires white you'll have to apply this colour with a paint brush.
Unless the subject I'm wanting to depict is something specifically special, like say for a relative, then yes I can see taking the route of home-made decals. Otherwise, for me, it's just not worth it.
regards,
Jack