Why are conventional and bait caster reel standard with right hand retrieve and spinning reels standard left hand retrieve?

I switch all my spinning reels to right hand retrieve, granted I’m left handed. I get comments all time when people fish with me about my reels being backwards or setup left handed. I simply ask them what hand they reel with on a conventional reel. Keeping the handle on the same side no matter if the reel is mounted above or below the rod only make sense to me.

Discuss.