Originally Posted by
canadiancow
Because that's not how airfare works.
It's also possible to buy a ticket for A-B and B-C that's cheaper than one A-B-C ticket.
For example I recently booked SIN-KUL-BKK-MEL, it was cheaper than SIN-MEL, SIN-BKK-MEL, and SIN-KUL-MEL unless I wanted 28" legroom on a LCC (makes rouge look good) and no lounge access.