I have heard "nohow" but have not heard "nowhen". My dictionary has "nohow", with two meanings, one labeled "US" the other labeled "archaic"
nohow ADVERB
1 (US) Used, especially in uneducated speech, to emphasize a negative.
‘they never executes nobody nohow’
2 (archaic) Not attractive, well, or in good order.
LEXICO
The answer to "why are they not real words?" is they have been used rarely or never. But if you like "nowhy", go ahead and use it yourself. Maybe it will catch on!
Think of "nohow" as a negative form of "anyhow". In fact I think all of your words are like that:
- Anywhre-Nowhere
- *Anywhen-Nowhen
- Anybody-Nobody
- Anything-Nothing
- Anyhow-Nohow
I said I have not seen "nowhen". Well, maybe I have seen it (along with "anywhen") in time-travel science fiction.