Ok explain me this

Like why do some Pokémon have the evolution while some don't? Like we got Lucario but where is riolu? We got Pikachu but where is pichu? And the worst part Raichu is gonna be a thing in pokemkon unite which is weird considering why they didn't add it to unite at first.. Like I get it Pikachu is more popular and they should give us an option to stay as a Pikachu tbh like syther and sizor...