In order for preprints to be considered permanent research objects that are able to be cited and used to demonstrate productivity for grants or job applications, readers must be able to cite them without fear that they will spontaneously become unavailable. Therefore, removal of preprint files is typically reserved for cases where there are significant ethical or legal concerns with the original paper.
Instead, most servers allow authors to post a new version of a preprint, which is in some cases displayed preferentially even though the old version can still be viewed and cited. Some servers enable withdrawal of preprints (similar to retraction of a paper) by posting a new version of the paper that is effectively a withdrawal notice. See details on servers versioning and withdrawal/removal policies in the Preprint Server Directory.