Humen is not the plural form of human because it comes into English through Latin. Words like man and woman become men and women because they are native English words. It’s similar to other -en plurals: oxen and children. Words borrowed into English don’t experience the same changes as native words do.
“Refrigerator” was borrowed into English from Latin and keeps the Latin spelling while “fridge” is a word derived from refrigerator by English speakers. At first it was spelt “frig”, but as time went on it merged with regular spelling rules to become “fridge”; similar to words like “bridge” or “ledge”.
Less than 25% of all English words are native English. This means that over 75% of English vocabulary has been borrowed from other languages. English has a long history full of contact with many other tongues. Latin & Greek give us 39% of English words, French gives 29%, and 7% comes from elsewhere.