I don't know why there is so much naysaying about this. There are many people who mix their own feed with great success and show how they save money. There are many who ferment the feed for nutritional and feed reduction purposes. I don't do it because I'm too lazy.
If you've reached the end of this thread, and still don't know why there is so much naysaying about this, then I have somehow failed in my efforts to educate with hard figures.
For the majority of people, the majority of time, they lack the education, experience, research, facilities, and economies of scale necessary to compete economically with "the big boys" in the crafting of a nutritious, economical feed. To mitigate the first, they often rely on internet or youtube personalities (i.e. the aforementioned "Garden Betty" whose own knowledgebase is clearly deficient), and for the second, they generally don't pick up a calculator (or they are simply math impaired - something else I've seen on full display in some of the youtube feed videos).
Nor do most owners have truly healthy birds to compare to. If all your birds look like teens and adolescents raised on a diet of doritos soda white bread and peanutbutter, the whole flock looks healthy (when compared to each other).
In short, they simply don't know any better, and choose not to learn.
That's not to say feed can't be mixed at home, just that it requires effort to do so well, and it can only be done economically under unique circumstances not enjoyed by the majority of BYC posters.
None of which has anything to do with fermenting, a process with its own myths surrounding it (ACV is another good example of this). At least ACV and the various ferment methods are useful to some people in some circumstances.