Because It's not a simple matter of the file exists or not.
Think about these 3 real case examples:
- A not gracefully end of the tcp connection can occur.
- if the file is stored in a Pen Drive, the user can unplug the device in the middle of the process or a bad contact in the wires can lead to physical IO errors.
- If the user is connected via an USB Wireless adapter, if the user unplugs the device or a bad contact in the wires occur, physical IO errors can occur.
Many other unexpected situations can occur.