The reason is that it's hard to know how much to dose. I do know of someone who has used it for years as a wormer for her birds. She has a website where she explains what she does with the paste and I have used her method one time, but since then decided that Eprinex Pour On is probably easier to dose and with no real withdrawal, at least that I infer from the "no meat, no milk" withdrawal, since its a cattle wormer.
I used the paste. I squeezed out a one inch strip and put it in a pint jar of hot water. I let it sit overnight to dissolve and poured it into a one gallon waterer. Filled the waterer and swished it good to mix and that was the only water source for my chickens for 3 days. I confined them to the barn. HTey were not happy, but it was raining, so they would have been hunkered down in there anyway.
I did a 4 week withdrawal time, which worked good as they were not laying much anyway, so it really turned into a 6 or 7 week period.
yes thats why I bought it and I had thought I looked it over pretty good to make sure there wasnt anything harmful in it.
speaking of cheaper----I just bought some really expensive cat litter--I bought a new bag of oyster shell for the chickens and one of the cats decided to get inside the bag and use for a handy dandy litter box rather than qalk the xtra 20 steps to the bathroom where her litter box is