Pikachu is one of Pokemon's most beloved characters, and possibly one of the most recognizable characters in the series. In fact, even non-anime fans have seen this face or know the name in one way or another, even peripherally. As Ash Ketchum and the gang continue their adventures throughout the anime, Pikachu is forever at his side, quite literally.
Fans would have noticed that Pikachu is always outside its Poke ball, never to be seen going inside it. In the very first episode of the anime, Pikachu stubbornly refused to even acknowledge the Poke ball. The question is, why is Pikachu never inside its Poke ball?
Ash and Pikachu's Relationship
Pikachu was Ash's starter Pokemon, given to him by his former owner Professor Oak. Due to his lateness in receiving a Pokemon, Ash was stuck with Pikachu as opposed to selecting him. Pikachu proved to be a spunky little creature, electrocuting Ash and refusing to go inside the Poke balls he threw at him. After an initial rocky start, the two became very close companions, with Pikachu always seen perched on Ash's shoulder. The two go on countless adventures together with Ash's friends and fight foes such as Team Rocket, who always try to kidnap Pikachu.
The Poke Ball
Poke balls are used to contain all sorts of Pokemon. Despite the varying sizes of Pokemon, they will be readjusted accordingly to fit inside their Poke balls. Junichi Masuda, the producer of Pokemon Sun and Moon, humorously stated in an interview that it is "a very comfortable environment" inside the Poke ball.
He equated the inside of a Pokeball to luxury suite in a top class hotel. It seems that once the Pokemon are safely contained in the Pokeball, they are well-comforted and living an extremely spoilt life. Whether other Pokemon can interact with each other inside the Pokeballs is unknown; if not, it must be incredibly lonely for some Pokemon at times.
So Why Does Pikachu Remain Outside its Poke Ball?
A theory for Pikachu's preference for the outside world is that he may not enjoy the environment inside the Poke ball, no matter how comfortable it may be. In the event that no other Pokemon can interact with each other inside the Poke ball, it is possible that Pikachu does not like being isolated in there. Why be all alone inside a Poke ball when he could be interacting with his best friend Ash? If the Pokemon are isolated inside the Poke ball, then perhaps it is better for Pokemon to be outside at times to interact with other species of Pokemon in the real world. The Pokemon Camp option in Pokemon Sword and Shield allows for this kind of interaction.
Additionally, Pikachu is also very close to Ash. Perhaps the reason he is never inside the Poke ball is because he simply wants to be near him at all times. Ash has proven in the very first episode that he is loyal and dedicated to Pikachu, protecting him from Spearows and getting attacked in the process. Since then, the pair share a very close bond. It is possibly this bond that makes Pikachu want to be physically as close to Ash as he can be.
It can also be argued that Pikachu's loveability is the reason he remains outside the Poke ball. Pikachu became a fan-favorite character in the games and anime, therefore perhaps Pikachu is always on screen is because the audience wanted to see him. It would not make sense for one of the favorite characters to be constantly contained in a Poke ball, out of sight.
Source: Kotaku