Not sure really on this question but by baby Pokemon are you like kinda referring to this?;
Riolu (Japanese: リオル Riolu) is a Fighting-type baby Pokémon.
If so, according to Bulbapedia;
Baby Pokémon can be classified as such by these four rules:
1. Can be obtainable by hatching an Egg.
2. Must be the lowest form in a family chain.
3. Must be able to evolve at least once.
4. In Undiscovered Egg Group.
^^More Similarities I suppose :P
Riolu fits those categories perfect so I suppose that's why? ;~;
Anyway, yeah most baby Pokemon were introduced after their first stage evolutions, with the exceptions of Riolu and Togepi.