Why are some species (pichu, riolu..) classed as “baby pokemon”? Why are they different from ‘real’ baby pokemon? After some time they wont exactly be babies anymore right? And why cant they breed? (sorry so many questions)

Quite all right! These are all very good questions.

To answer, baby pokémon are more or less equivalents to larval stages of certain animals. While they’re certainly more functional than their animal equivalents (to the point where pokémon can engage in battle, problem-solve, and do much more that a larval insect or amphibious tadpole can’t do), they’re still merely infertile juvenile stages, and thus, they can never gain the ability to fully mature and breed unless they evolve. This is in contrast to many other hatchlings, which retain their forms even to sexual maturity, so a puppy growlithe is still the exact same species and form as an adult growlithe. It’s just that the puppy needs to grow into an adult before it can breed (as oppose to evolve in order to reach that point).

Thus, in other words, baby pokémon are considered as such because they’re essentially larva. They can’t breed, meanwhile, because they’re infertile until after they evolve and, thus, develop functional reproductive organs during evolution. For this reason, a three-year-old pichu will still be considered a “baby pokémon,” as this is just shorthand used by trainers and researchers to denote a larval pokémon that has yet to evolve, not literally a hatchling.