...Is that you have to finish writing them before you can play.
A generic RPG doesn't define: Campaign, setting, genre, theme, power level or a discrete set of unified abilities. By "discrete set of abilities", I mean a unified, coherent, appropriate and complete set of abilities for your campaign, setting, genre, theme and power level.
I think generic RPGs should have books that define this "second half of a generic RPG". A unified setting, genre, theme, power level and discrete set of unified abilities, all in one book.
Or generic RPGs will continue to be ignored by 99% of the gamer population.