Because the airline thought of your brainwave before you did! Airlines don't want you to undercut their standard excess baggage charges this way. I couldn't find a SQ charge on their web-site but I note that BA charges for their long haul flights GBP32 for an extra bag up to 23kg and GBP72 for the second (to 9th) extra bag.
Surely you can't buy a second seat long haul ticket for GBP32!