Knights and Masters however, are free to leave if they wish. They have earned their right to leave the Order if they wish, and for any reason. But however many Knights and Masters do not throw away their duty and training lightly, as they are treated as disgraces. Many other Jedi feel that no one who has achieved the rank of Knight or Master has no reason to leave the Order, since it was proven that becoming a Knight was their destiny, written by the Force. Even if that is not the case, many Jedi feel that Knights and Masters who leave are shirking their duty to the Republic, are betraying their friends and family in the order, and are suspected of turning to the Dark Side.
But however, the ultimate reason that Jedi don’t easily leave the order is because if they do, there is no coming back. Leaving the Order is something that the Jedi Council does not take lightly, and if you chose to leave, then you won’t be allowed to return, for any reason, or under any circumstances (With few rare exceptions, as is the case with Revan)
The second reason is that after you leave, the Jedi Council will keep tabs on you. The Jedi view any other non Jedi Force user (referred to as “Gray Jedi”) as a threat to their order, regardless of said Gray Jedi’s views or use of the Dark Side. Despite what Obi-Wan said, the Jedi believe that if you are not with them, then you potentially against them. And ex-Jedi and Grey Jedi are always dealt with by the Order with the thought of them being a threat in mind.
That is why it is so hard to leave the Jedi. Despite what the Jedi say about attachment, the Jedi do make friends and family in the Order, and therefore do not want to leave their family forever, and have that family treat them like a threat and deserter.