Hahahahaha that's what I first thought you asked, but then I said, no maybe is asking about the title
Well then, the answer is, why not? hahaha I always think these Pokémon have some things in common, like their power, personality, etc, except for Lugia. Lugia was added at the end. When I put Deoxys, Mewtwo, Lucario, Zoroark, I thought, the second generation is missing, so, I didn't feel good, and decided to put one of each generation
I have many other drawings that include exactly one of each generation too, at least the generations that existed at the time they were made, but they were not planned like this one.
So, I just decided to put one of each generation hahaha
Hahahaha, I think there's no similarity phisically, but maybe is something about their roles, like in the movies, or the videogames, I don't know And maybe the people that you say they think these Pokémon are similar too, has different reasons than me.