Some Pokemon are on pokedex yet you cannot get it?

Question

Why is the Pokémon and pokedex like this? they appear on dex yet you can't get it? LEGITLY is what I mean. such as jirachi?

Answer 1

These Pokemon are in the Pokedex because they are Pokemon, so they have to be in the Pokedex. Also, the Pokemon that you are (probably) thinking of are event Pokemon, such as Jirachi. The only Pokemon that are not obtainable as if now are Volcanion and Hoopa.
Hope I helped!

Answer 2

The only Pokemon that are not obtainable at the time of this post are Hoopa and Volcanion. And yes, there are Pokedex entries on them

Hope I helped!