First, with the Monkey King as the master elder brother, Zhu Bajie's force value is relatively low. On the way to learn the scriptures, the Monkey King has always been a scout in front. If he meets a monster, the Monkey King will collect it. If the monster's martial arts are too high, the Monkey King will go to Heaven for help, or go to Guanyin for help. To put it bluntly, the Monkey King almost monopolized the whole Buddhist scriptures, and Tang Priest had solved some things before Pig Bajie came to the Monkey King.
So it's not that Zhu Bajie's force value is low, but that he has no chance to show it. Besides, according to Zhu Bajie's assumption of serving in heaven before, he also knows what these monsters are. After the previous monster came down to earth, he also understood that learning from the scriptures was only a form, so he didn't do his best.
Second, it is said that Pig Bajie left half of his mana to protect the whole village when he left Gaolaozhuang. I have seen a movie before, which tells the story that Pig Bajie was demoted as a pig head and lived in Gaolaozhuang. The most telling thing is the relationship between Zhu Bajie and Gao Cuilan. Although Gao Cuilan is only a mortal, Pig Bajie really treats her sincerely. Pig Bajie agreed to Guanyin's proposal and listened to Tang Priest's suggestion, leaving half of his mana in Gaolaozhuang.
If nothing else, it is because this is Gao Cuilan's hometown and he wants to protect it for her. After all, the reincarnation of Gao Cuilan also lived here. Protecting Gao Lao Zhuang means protecting the peace of Gao Cuilan's reincarnation.
Finally: If Pig Bajie is really a person with the lowest force value, Guanyin and Tathagata will not let him learn Buddhism with Bai Piao!