The answer is very simple. (Good try, all of you out there).
The minimum requirement of 5 GCE "O" Levels including English, Maths and a Pure Science subject in one sitting is laid down by the CAAS (Civil Aviation Authority of Singapore) who is the licencing authority of all types of pilot's licences in Singapore.
So, really it (the minimum requirement) is not SQ's invention.
CAAS has the final say, therefore SQ (and any one who wants to obtain a CAAS Commercial pilot's licence or ATPL has to comply.
Amen.