Why the rule has been not made yet about the voice of "g" and "c"
What rule exactly are you referring to? Can you be more specific?

Regards
There are two issues: (1) heuristics used in phonics (2) phonetics

g: /g/, /dʒ/
c: /k/, /s/

/k/ is voiceless, whereas its counterpart /g/ is voiced.
/s/ is voicelss; /dʒ/ is voiced.

Velar consonants get softened (or voiced) before front vowels esp in derived-words: cf. electric vs electricity

Given this background knowledge, what is your concern?