Many times I have been asked by students: “Teacher why is it like this in English?” To which I have replied: “Because it is English!” I am reminded of a school friend from Italy who was confused with the construction of plurals. “If it is house and houses but mouse and mice, why ‘it no hice’?” We all laughed but there was the start of my love for languages.
It starts with the origins of English as a language made from other languages and languages which have evolved. In the case of mice and mouse, it is because Old English also called Saxon English, sometimes made the plural in the middle of the word rather than at the end.
So the middle plural changed mouse to mice and the plural at the end of the word changed house to houses.
The English language has changed so much that you can actually study courses at university to learn about how words have changed and why they changed. Personally I find it a fascinating subject but it only affects my teaching to explain why English is structured the way that it is.