Question About Ash's Pokémon's Capability of Mega Evolutions?

I knew that Charizard, Pidgeot, Heracross, Sceptile and Glalie have been capable of their Mega Evolutions. Ever since Ash's Lucario have already Mega Evolved for the first time, while Ash's Gengar had already Gigantamax for the first time instead of Mega Evolving, I thought that Ash should've Mega Evolved Charizard, Pidgeot, Heracross Sceptile and Glalie. So, you don't think that being fully evolved already means that Ash's Charizard, Pidgeot, Heracross, Sceptile and Glalie is capable of Mega Evolutions, do you? Why? Please let me know, thanks.😉