>>16378612Megas were a popularity contest combined with the desire to make some other mons more viable.
Yeah, Garchomp, Tyranitar, and Gengar were the last pokemon that need a buff, but they also got to take awful Pokemon like Kangaskhan, Mawile, and Ampharos and breathe new life into them. If Megas were only given to Pokemon no one cared about, they wouldn't have been able to hype it up nearly as much.
Plus, we'll see more megas soon. Likely the same kind of mix of "don't need it" and "desperately need it".