For the same reason that IKEA doesn't sell cars. For the same reason why people dealt with Mux In A Minute (MAIM) in spite of its halfway working, halfway broken oWoD plugins.
Issa lotta work what nobody has bothered to do yet. All the bits have to fit together just right, and people want their own house rules which will break those bits.
I think Alzie has some systems which are just stat/value setups where you roll and interpret the dice yourself, though. And evaluate the character sheet yourself for errors. Set stat, roll stat, see stat, lock set stat, there's a game.