The reason that we don’t need to breathe whilst sprinting over short distances is that we don’t need to get oxygen to our muscles as there are enough substances already in the muscle (e.g. glucose/glycogen) to make them contract and allow us to run.
Breathing would therefore slow us down, and not breathing allows us to concentrate fully on the leg muscles.
If we sprint for more than a short distance our muscles do need oxygen though so you would have to breathe.
Hope this helps, if you have any further questions then please ask 🙂