This is not original, if you don’t know it 
The request has come up on this board many times, in the last 15 years.
If I recall, it would be an issue with reworking animations and such with the other hand. It would not be as simple as flipping a switch. If the game is MADE so you can choose, then it is easy; to do it afterwards is more difficult.
Even on the European forums:
https://eu.forums.blizzard.com/en/wow/t/why-does-the-game-assumes-my-dominant-hand/96513/27
By the way, I have HORRIBLE handwriting (with my right hand.) But I am supposedly “right handed.” Being 53, I’ve always had this suspicion that I was “forced.” Of course, I can’t write at all with my left hand NOW. Another reason I’m suspicious is that I default to doing a lot of other things with my left hand, but am not really what you’d call ambidextrous either.
On the other hand, maybe it is best? I never even heard of “silver surfer syndrome” (lefties getting stuff on their hand, while “writing” (remember writing?
)
https://www.boredpanda.com/funny-left-handers-problems/?utm_source=google&utm_medium=organic&utm_campaign=organic
This one amused be the most, but I can’t post images:
https://static.boredpanda.com/blog/wp-content/uploads/2017/08/funny-left-handers-problems-15-598483c5d5096__700.jpg
See, because the left handed person would have the TEXT (about being left handed) on the cup FACING THEM (rather than a third party, who could read it)! So it kind of makes the point moot 