It has to do with anti-people trafficking laws John.
Not all countries are as liberal as we are in the UK (remember...no Divorce in the Phils either. Apart from the Vatican City, the only country in the world not to permit it).
Yes, I've experienced the frustration of these regulations when we wanted to send my step-daughter (then 13) home from the Phils back to the UK...where she'd already lived for 4 years with a settlement visa.
Her mum had already gone on ahead to sort out our house in the UK.
Imagine our annoyance when her mum had to return the Phils purely to accompany her daughter on the plane.
It would have been possible, but involved a long drawn out process with lots of forms, proofs and messing about. In the end it was easier for her mum just to come and collect her.
In the case of our British passport-holder son, there was no problem him returning to the UK unaccompanied...which he did aged 9 years, because he had a BRITISH passport, so the Phils authorities had no jurisdiction over him.
For the current options open to to you...information here:
http://www.dswd.gov.ph/faqs/travel-c...ce-for-minors/