The answer to why Mob stopped working for Reigen is simple. Shigeo wanted to focus on other things. Mob’s life became busier than ever. He was given more responsibilities at Salt Middle School. Hence, at the end of the final season, Kageyama Shigeo or Mob, is no longer a part-timer at Reigen Aratak’s consultation Office.
Mob started to work for Reigen at a young age. With the hopes of finding answers to his questions regarding his psychic powers and experiences, he visited Spirits and Such Consultation Office believing he will meet a psychic like him. There he met the man who will later on change his life, Reigen Arataka.
Who Is Reigen Arataka?
Reigen is an average human being. He is definitely not like most of the characters in Mob Psycho 100 who possess psychic powers. Reigen is the owner of Spirits and Such Consultation Office, a former one-man agency. He is best known for being Mob’s master and boss.

But before becoming the deuteragonist or the second most important character in the series. Reigen Arataka was just an average salaryman. Before meeting Mob, he was just a simple person who was trying to get by. He even admitted to one of his clients that his saddest moment with his parents was when he didn’t have anything to eat during sports day because he forgot to inform his parents. That’s it. He didn’t have much of a struggle growing up, aside from finding his true passion in life.
Reigen represents most of us. When we get tired of certain things, we try to find other things that would add excitement to our lives. Thus, Reigen might not be as powerful as Mob, but he is sure smart. In season two episode seven of Mob Psycho 100, Reigen admitted that he was always the curious type of guy.

The Spirit and Such Consultation Office was simply built because of an advertisement he saw in a magazine. From then Reigen started his exorcism business by being a self-proclaim psychic. Without any powers to help his client, Reigen was technically a con-man, who only uses Salt to exorcise spirits – which were never effective by the way.
As he started to get tired again of his chosen career, Mob enters the office and willingly showed Reigen his psychic abilities. Reigen hiring Mob first started as one of his schemes in order to further flourish his career as a pretend-esper. However, despite all of his malicious doing. Reigen was no doubt a good man from within who appreciated and only wanted the best for his mentee, Shigeo.

But, after years of being the esper who does it all, Mob finally decided to temporarily part ways with Reigen.
Did Mob Quit Being an Esper?
No. Mob will always be an esper regardless if he works in Spirits and Such Consultation Office. After the ???% Arc, the relationship between Mob and Reigen became more genuine and stronger. As both of them finally confessed their true feelings and perspectives.
The dark side of Mob who he had always fought from within knew all along that Reigen was a fraud. He didn’t actually have any powers, but Mob refused to believe this because of his kindness. When Reigen finally confessed to everything, Mob got to appreciate his beloved Master more.
Because of Reigen’s honesty despite feeling so ashamed of himself, Shigeo successfully became one with himself. Their heartfelt conversation during the last episode of the series paved way for them to grow individually.

But even with their closeness, Mob still decided to quit his job at Reigen’s consultation office. In season two episode twelve, Tome Kurata, one of Shigeo’s friends and the president of the Telepathy Club, was the newly hired secretary of Reigen.
Six months went by fast, and so many have changed since the ???% Arc. As Mob finally came to terms with himself, he lived a happier and livelier life as the new vice president of the Body Improvement Club. Moreover, he has decided to focus more on his studies to better prepare himself for high school.
Even without the presence of Mob, Reigen wouldn’t go back to his old days when he continuously tricked people by pretending to be a psychic. Hiring the former Claw member and psychic, Serizawa, enabled Reigen to not close his consultation office and continue serving those who needs their help.

Will Shigeo someday work again for Reigen? Probably. However, there is no assurance of that as the manga and anime series of Mob Psycho 100 officially ended last December 2022.
But, with their unbreakable bond, the chances of them working together again in exorcising evil spirits using Mob’s overkill psychic powers and Reigen’s ultimate Salt Splash technique, will never be impossible.
Read More: Who Controlled the Divine Tree in Mob Psycho 100?