Many languages borrow words, not only English. It doesn’t mean, however, that they are left unchanged in their original version, complete with plural and other forms. That’s because languages have their own rules and traditions. Most of the native speakers usually don’t know the language where the word came from, but know (or feel) the rules of their language. In some cases pronunciation, spelling, forms, and meaning do get preserved, but very often they do not. Like it or not, it’s a part of a natural process of language development.
Say, in my native Russian there are many of the words that have become international. Say, “virus” is “virus”, but you can hear “virii” may be among medical students, at best, and never “fora”. It just doesn’t suit general style of the language, that’s it. Just because the word is borrowed, doesn’t mean its plural should. Maybe even the opposite. When the word from borrowed becomes truly appropriated, it creates forms natural for the host language.
(By the way, MS Word autocorrected “fora” to “for a”, and underlined “virii” as an error. Don’t you think a number of users of “forums” and “viruses” would grow vastly from this alone?)
You see, I’ve never had a chance to study “proper” Latin or Greek. Just a few words here and there as a part of other subjects. I did study English as a foreign language, and I did know irregular verbs exist, and even had to memorize a few dozens, but all I knew about “irregular nouns” is that a few exceptions exist from a general “add ’s’ for plural” rule. Say, for some weird reason “child” becomes “children”, and “mouse” becomes “mice”, but that’s nearly all. I work with index files for years, talk to other programmers at work, and only last year did I find out – from Tek-Tips, by the way - that they could be “indices” and not ”indexes”; and only reading this thread I heard that this is a preferable form. America being a heavily immigrant country add to the picture. Many people around, even speaking a relatively good English, create plurals using familiar rules.
And one more note. As sleipnir214 noted, English “borrows words from every other language on the planet”. But for some reason, it tries to use original plurals/pronunciation only from some of them. I don’t shudder, I internally laugh when I hear common pronunciation of “sputnik”. Did you know it’s more like “spOOt-neek”? Did you know the plural was something like “spOOt-nee-kee”? Not that I actually want to hear it in English; “sputniks” is OK with me. Because it’s English now, not Russian any more.