Well, it’s just anime logic. Nothing makes sense in the anime, it really doesn’t. Moves that always hit like Aura Sphere miss. Charizard gets thrown around by a Chikorita. Charizard knows Tackle (there are so many things wrong with this. It’s an illegal move and it’s an awful illegal move). So yup. That’s why.
I will point out that Bianca traded her Shelmet for a Karrablast and got Escavalier, and Jessie traded her Pumpkaboo and it evolved into a Gourgeist, so really, the anime is also inconsistent (we’re looking at you, Pikachu).
And apparently you could find trade evolutions in the wild for a while now.
So it’s a mix of anime inconsistency and anime logic.
Hope this helps! :)