Why aren't introns found on the ends of pre-RNA?
RNA splicing refers to a certain kind of RNA processing mechanism which leads to the excision and exclusion of some regions of the primary transcript. You should note that this is not the only method by which such a thing can happen; Endo-ribonucleases can clip the ends of the transcript and this happens in the case of tRNA-processing. But as "splicing" happens it is essential that the excised region be in the interior of the transcript. See the figure.