Because while the translation has the same meaning, it's a completely different expression. This happens from time to time even for languages as close as English and German.
The literal equivalent of "to miss s.b." is "jemanden (Akk) vermissen".
(You can also say "Ich werde dich vermissen", BTW).
"fehlen" means "to be missing, to be lacking, to be needed"; there's no direct English equivalent. "jemandem (Dat) fehlen" adds a dative object as indirect object (the person who is affected by the action, the person who benefits or suffers from it). The dative is often used for this purpose. So a more direct attempt at a translation would be something like "your state of absence makes me suffer", but of course one doesn't say that, one says "I'll miss you.".