Why must my 1 Magnemite always get 2 more when I'm in Azalea Town?
Not sure what you mean by this.
I think it's to make the game fair and simple enough. If you had to capture and raise 3 Magnemite to finally evolve them to Magneton, there would possibly be even fewer Magnetons out there. Same with Metang/Metagross and your other examples. It could make some pokémon too tedious to bother with.
That's why it's nice that we have manga, anime and other media, to expand upon the rather limited laws the games give us. Venustoise and Zapmolacuno <3 So in ways, Kyurem's fusions were pretty old news.