Did you catch the point of the comment : "Erasmus of Rotterdam, responding to Martin Luther's question, "Why do you remain in the church?", is supposed to have replied, "I put up with this church in the hope that it will become better, just as it is constrained to put up with me in the hope that I will become better." that it is Erasmus making the comment, not Luther. 
Luther left, true enough. Erasmus stayed. And the reason he gives is a good one: ""I put up with this church in the hope that it will become better, just as it is constrained to put up with me in the hope that I will become better."