Because the keynesian multiplier does not exist … ? Some book I recommend for you.
1/ The Failure of new economics (chapter 11 “The multiplier”; see especially pages 137, 139, 146, 150-151 and 192).
2/ Dissent on Keynes (chapter 4 “The Myths of the Multiplier and the Accelerator” pages 78-79).
3/ Money Bank Credit and economic cycles (chapter 7 “A critique of Monetarist and Keynesian theories”, pages 559-560 and 562).