Unfortunately, most of the time when heading to MNL via HKG, I am either on UA itinerary with SIN as main destination from the US, thus a separate SQ ticket from SIN-MNL, or an award ticket. I don't think singaporeair.com has any trouble issuing HKG-SIN-MNL in one ticket, although the price indeed is the sum of the regular costs of each segment purchased separately, which is expensive. Anyways, given PR, CX, and 5J's more convenient service between HKG and MNL, I don't think there is competetive reason at all for SQ to be less restrictive and lower the cost for this kind of routing.