For me personally, even though I have plenty of Remilia pictures that would perfectly as a profile picture, I have developed a sort of attachment to this one and replacing it would hurt my heart a little. (Although its fair to mention that I have a habit of replacing my discord profile picture every two months or so.)
As Felis-Litch said, it also has to do with the "internet identity" people create. The more time one has a certain profile picture and username, the more they start identifying themselves with it and it just becomes harder to change.