that’s like saying, why do Demon Hunters need horn or tattoo options?
it’s called player customization options. Just search google or this and the old forums, you’ll find forum posts going as far back to 2010 asking for tails, many with thousands of replies and such 
Here are these more recent post:
and
https://us.forums.blizzard.com/en/wow/t/give-worgen-tails/504127
Or this classic post of mine:
https://web.archive.org/web/20161127203431/https://us.battle.net/forums/en/wow/topic/20743576065
That spanned multiple posts and reposts and had over 200 pages worth of over 5000 replies on the old forum in regards to wanting Worgen Updates and Tails 