Because when you delete a file you don't actually get rid of the data you just remove the 'pointer' that tells your computer where it is and that the space can now be re-used.
When a file is written, all its contents are written to the disk and an entry in the File Allocation Table (FAT) inserted (this tells the OS where the file is, its size etc). When the file is deleted, only the entry in the FAT is removed.