Mr Horgy...
Having looked at the updates I referred to earlier, I just wonder if its a case of avoiding confliction with landing traffic on RW12... in the case of a go-around or missed approach being necessary off a 12 arrival, a left turn off 13 could be a bit hazardous... As Mr Hughes suggests, I guess it also ensures that a departing aircraft is well clear of all that cumulo-granite to the North of AGP...
I know what you mean about track miles and time spent getting nowhere other than up though!