That all players that take part in the competitions will get the Mega Stones before everyone else.
If you want them early, you have to battle :P
Eh, that's alright, I'm sure my in-game team will do fine enough.
Incineroar: Say what now?
Me: Oh don't be like that, you guys do perfectly fine in the Battle Tree.
Mimickyu: Almost every match we have in the Battle Tree is a close call! And that's against AI.
Magnezone: Bzzt, we'd be slayed in competitive.
Me: Hey, ya just got to battle for me to get the Mega Stones, no one said anything about winning.
*My in-game team just glares at me*
Snorlax: I wish I was dex fodder so I could spend all my time at the Poke Pelago...