The Users had a problem: the candidate host form, though bipedal and tool-using, was rather too large for optimal parasitization, until the stock had been bred down to proper size, the Users would have to make do with immature forms. And since this would mean discarding mounts every few years. 

inconveniently large breeding ranches would have to be maintained.

Still, the inconvenience was minor, especially considering that the brain case of a human child was of a perfect size to hold its host, once most of the brain had been eaten