It's the era of instant gratification. Many just don't want to spend the time to do the research. Though your statement about any info you need is published just isn't quite true. Plenty of bullet/powder combos are left out of manuals. Consider there are about 50 diff powders that are suitable in one form or another for most handguns, with 3-4 diff bullet wts, not manual is going to cover all that.
In times where common "book" powder become scarce, being able to switch to something else is valuable & wont be covered in any book.

__________________
NRA Cert. Inst. IDPA CSO