Because the hardest pull on the cleat and its angle is when the spi pole and hence the guy is on the forestay.
The guy pulls against the closed side of the cleat when reaching which is much more preferable.
When running, if the guy comes out of the sheet and you have a second cleat in the boat your still under control.
Plus you have to get it in the cleat somehow and so it has to have an opening!
This is why twinning lines have been found to do a better job, they hold the sheets where ever the spi sheet angle goes and can pull the sheet to the reaching position from within the boat so you don’t have to reach out of the boat to or go to the other side.