I was honestly just taught to accept that when I was a kid. It is just the way it is. In English, we call 14 fourteen, but we call 11 eleven. Why not "oneteen"? I asked my family why is 15 mười lăm and not "mười năm". They replied with "it sounds better". Sorry if this isn't exactly the most satisfying answer =(
It's okay, because every language fudges its own rules for the sake of making the spoken word feel better to say.
When writing the opening comment in this section, you should try to realise the number of inconsistencies in English! For me easy to say, because nor Vietnamese nor English is my mother tongue.