TRUE- that’s one of the flaws with a lot of stories. They think Mega Evolution is a power up that lasts forever and can be used by anyone. In reality, the reason why Mega Evolution works is a Pokémon and Trainer must have a strong bond and *usually* must be in battle. There are some edge cases in the Anime and the Manga where it’s possible outside of battle, but there’s usually a massive plot point happening (Anime: Kalos Crisis, Manga: Meteor about to destroy the world.)
And on the topic of Ash Greninja, the Bond Phenomenon is different from Mega Evolution (this is my opinion on Ash Greninja btw-). If any Pokémon was able to do something similar, it would’ve messed with the entire reason why the Bond Phenomenon was different from Mega Evolution.
Bond Phenomenon is when both Trainer and Pokémon have an extremely strong bond, and their passion is the same. Mega Evolution is when both Trainer and Pokémon have a strong bond and have two special stones.