Here's why:
* Oak's Role: Professor Oak is primarily involved in the beginning of the game. He gives you your starter Pokémon, helps you understand the basics of Pokémon training, and guides you on your early journey.
* National Pokédex Completion: Completing the National Pokédex usually signifies the end of the main storyline. Once you've caught all the Pokémon, there's no further need for Oak's guidance.
Where you might encounter him:
* Original Games: You might see Professor Oak in the Kanto region of the original games like Red, Blue, and Yellow, but his role is minimal.
* Spin-offs: Some spin-off games might feature cameo appearances of Professor Oak, but he won't be involved in the main storyline.
Let me know if you have any other Pokémon questions!