Its mainly because of three things.

1) Yes, mega evolution. I guess your right, that can be taught.

2) Yeah, new pokemon. Your right, you can  look on wiki. But really, you wanna look through pokemon, like, five hundred them. Its just better to know them.

3) Moves/Abilities change. There have been many new moves/abilities, and im sure you dont want to look through them.
[@KoL]