Becuase they cost like up to £10er for some of them big spanners. It would cost me a lot more buying sepratly. But i soppose i wunt need allll that stuff but still i migth one day (Y)
EDIT:
All you need;
BB tool,
Crank extractor,
Cassette lockringtool and chainwhip,
one spanner for pedals,
4,5,6,8mm allen keys
But it would cost up to £20 something for just that. And i can still think of stuff i would need really.