etymology - Why are plurals ‘*humen’ and ‘*Germen’ not conventional?

Studying English in school as a second language, I learned that human being would be the proper noun to describe a member of the Homo sapiens species, but it seems human is perfectly acceptable in English nowadays. It can even sometimes substitute (supposedly gender-specific) man where that was used in a gender-neutral way, e.g. in humankind, but not in *chairhuman, *firehuman etc.

The gender nouns man and (derived) woman, however, have their plurals as men and women, respectively, so why is umlauted *humen not acceptable at least as an alternative to the regular plural morphem +s which is used to form humans? The same question applies to some peoples or tribes, e.g. the Germans and Normans, not *Germen and *Normen.

The question even applies if human and man have different actual etymologies (Romance vs. Germanic), because perceived or folk etymology is often at least as important.

Also, does this effectively render these words gender-neutral, although they seem to contain the gender-specific morpheme man (in singular)?

Answer

Words like human and German are not from man and do not contain the (Germanic) morpheme man, as you say. Only the morpheme man is properly pluralised as men. See the list of words in Tchrist's answer for an overview of which words are from man and which aren't. In human and German, we're dealing with the Latin suffix -anus, which means something vague like "having to do with x", shortened to -an in English.

Cf. Republican (same Latin suffix), Qur'an (an Arabic morpheme): we don't say two Republicen were reading their Qur'en.

The true etymology is very important, because folk etymology is not very common in general and hardly ever accepted by the "writing classes". So it does not often become popular enough to displace the original.

Further, there would be little difference in pronunciation between human and humen, because the last syllable is unstressed; folk etymology normally originates in speech, not writing, which means that this plural would be much less useful to speakers. (In women, a trick was performed by having a differently pronounced first syllable to distinguish between woman (/wʊ-/) and women (/wɪ-/), which words would otherwise be pronounced (almost) the same.)