>>31738321What? That makes perfect sense.
Game Freak recognizes that by making Legendary Pokemon have higher base stat totals, they are generally much stronger than other Pokemon- this is a by design and is a themeatic choice.
The post-game battle facilities have balance and difficulty in-mind. You and your opponents are supposed to have access to the same Pokemon. If 670+ BST legendary Pokemon were available to the enemy, that would seriously encourage/pigeonhole the player into also using 670+ legendary Pokemon.
This would trivialize every battle that isn't vs other 670+ legendary pokemon, as regular Pokemon can't be expected to survive Mega Ray, Primals, Mega Mewtwo etc.
The best thing they could do to appease someone with your legendary sentiment without ruining the experience of other players would be to have a "strong legendary" tier where only those Pokemon are allowed. However this would get stale quick, as there aren't a terribly large number of Pokemon who fall in that category.
You'd keep fighting the same Pokemon repeatedly, (you already see them enough with the normal roster,) and every player tackling the post-game facility would just run Mega Ray/Primal Kyogre/Groudon.
Sucks that a personal favorite of yours can't be used in this mode, but it's for a good reason.