-1
Since u're saying why legends can be reroll while not shinies, oh well maybe cuz we only can catch legends 1 time if not counted that recatch and they are not tradeable.
As for shinies they can be caught at wild if u got the luck and tradeable, if adding the rr for shinies i can think of 1 way make em untradeable which none of the players want in this game. If added shiny rr and shinies still tradeable the market will be falling apart.