How do people no get the difference ?

I know it's no big deal, but i've always wondered why people have difficulty with the Your and You're thing.

My first language is french and yet I still get it and it seems very easy to me. Your : Your thing, possesive You're : Short version of "You are".

I know it's not a big deal, but putting this here might help at least one person to make the difference. I am just questionning how come people don't get something so easy ?