Because practices implicitly assumes that merit === talent, and that natural talent cannot be hidden by extenuating circumstances such as social pressure away from a given trade or poor access to learning resources.
Also, a meritocratic system only requires the "winners" are chosen on the merit of their work. It will never be possible to eliminate all disadvantages which might prevent somebody from achieving some abstract concept of potential.
Like a blind person can't be made to see, but screen reading software and braille text is still better then not having those things.