They do if we want them to. Take it up with Walter Benjamin’s ghost if you don’t like it.
It’s ok in English to say ‘I love the shinkansen in Japan’ or ‘I love that one shinkansen, which was it… oh the Nozomi.’ Because shinkansen is meant to he specifically Japanese and isn’t integrated into English, ‘I love shinkansens’ sounds clunky, and, sure, many would opt for ‘shinkansen trains’ or ‘Japanese bullet trains’ or something. But none of them are wrong.
Loan words in English are often moving between or straddling various levels of integration.‘Toyota’ is integrated to where ‘Toyotas’ is common. ‘Sukoshi’ comes in as a clumsy loanword with American specific use and pronunciation, i.e. ‘skotch.’ Pronouncing ‘karate’ correctly makes you look pretentious similarly to how saying ‘fillet’ the way Americans do sounds pretentious to Brits who don’t leave off the t.
Further, it’s incredibly rude and condescending to assume you know what I think about how translation works.
I can tell what you think about how translation works by the way you write. If you think that's rude and condescending, I honestly don't care.
Anyway, "hiragana" is not a loanword in English at all. When used, it's treated as a proper noun for something that's only in a foreign language. So nothing you wrote about loanwords applies.
Hiragana, katakana and kanji are untranslated terms that are included in major English dictionaries. They are loan words. It is common for less-frequently used foreign words to not be inflected as plural if there is not a specific plural form from the parent language. This may be confusing in some contexts and a translation that includes a plural noun to help provide that context can be useful. However, in a poetic or literary use, sometimes intentionally not inflecting a plural is a choice.
Being rude and condescending is an inappropriate tone for this site which should be centered around discussion and curiosity. Further, you’ve made zero substantial argument for your position, contradicted yourself and failed to make a case.
No, it doesn't. It means "Which kanji characters became which hiragana characters".
Words do not directly translate between languages the way you think.