Because in the Hindu tradition of the time (which you can read about here), those who became holy men or sages followed a path of renunciation. At a certain point in their lives, they would renounce what was called "householder" life - a life of marriage, family, work, etc. - and go live in the forest where they spent the rest of their lives in study, meditation and other spiritual practices.
Siddhartha's father did not want his son to become one of these renouncers, so he made sure his son's life was as perfect as possible, offering everything wonderful and nothing negative or upsetting. The father made sure his son was shielded from anything that would make him want to leave the kingdom.
Siddhartha begins looking for enlightenment initially by looking for external guidance from organized religion in the form of Brahmins, Samanas, and Buddhistsiddhartha leaves the Brahmins, the Samanas, Gotama, and the material world because he feels dissatisfied, not because an external source tells him to go
aap chud jaao mein kahu toh xd