Why do words such as "feet", "geese", and similar words do not have an "s" on the end as most other plural nouns in English do?  If you think about it, it is amazing how our English language have many forms.  According to Cyrstal, those nouns are "exceptional plurals".  He also states that there are only about a hundred forms of them.   Crystal further points out that there is actually not an answer to this question, but he notes that one can see that this "distinctive form" can be traced back to Old English or Germanic form.   For instance,  Crystal states that use of /n/ as a plural marker was truly a sign of an important feature of Old English nouns, and several nouns show this feature in Middle English such as doughtren (daughters) and sustren (sisters), both found in Chaucer (200).
    Perhaps, we also may consider prescriptive grammar being the reason why this standard has been preserved and not died out. Prescriptive grammar begins during the time period in which our society is looking for order since the form of Johnson's Dictionary.  That means spelling and lexicon is now being dealt with in a systematic way in which everyone can refer to.  Therefore, the leaders of grammar decided upon having a standard for certain words, and those where the ones that decided to adopt.  Today, we categorize those nouns as traditional grammar.  And Crystal says that traditional grammar is when the rules of what we call 'correct' grammar were first drawn up. (78).  "It was a time when the subject was debated at length, with philosophical, logical, aesthetic, historical, and occasionally linguistic reasons proposed for adopting one position rather than another" according to Crystal ( 79).
    In our lanuage,  sometimes there is not a direct answer to a question as Crytasl states above, it's simply the way it has always been.