While the existing answer has merit, I wasn't satisfied, so I did some research of my own.
Something to be noted is that most Ferengi we see in the show aren't speaking English; instead, what we hear is the product of a Universal Translator. This device doesn't always translate foreign words (for instance, Klingons can occasionally be heard speaking Klingon as well as translated English); there is a question on this site about that, but the answers don't seem to come to a consensus. The important thing to note is that there are cases in canon of species speaking their own language and not being translated.
So, the question remains, why doesn't the Universal Translator translate 'human' correctly when coming from a Ferengi? I found a Reddit thread that asked this same question. The most convincing theory presented was that 'hoo-mon' is a Ferengi word, and the translator does not translate it.
There is no direct proof of this, but there is a similar known Ferengi word 'DaiMon', which translates to 'Captain' (Memory Beta also lists a rank of 'GuiMon' as equivalent to an Admiral). Based on its spelling, it seems safe to infer that DaiMon is some sort of a compound word; in the aforementioned Reddit thread, the user BoozeMaster speculates that it could mean
either "Head Merchant" or "Lead Negotiator"or somesuch as the Ferengi don't seem to have military ranks per-se.
Again, based on Ferengi culture, this makes sense. HooMon, then, could be a Ferengi word that means (again, as proposed by BoozeMaster)
"Poor Negotiator" or "Bad Merchant"
If this is true, then when a Ferengi seems to say 'hoo-mon', they aren't trying to say 'human', they're using a Ferengi word that has come to be used synonymously with 'human', and is actually a cleverly veiled insult.
As for why the translator doesn't try to translate 'hoo-mon', I don't see how it could; if it translates the word to 'human' it loses the insult, if it translates the word to 'bad merchant' it loses the clever wordplay.