Because its not about being able to do everything in the law perfectly (grace covers where we fall short).... its about guarding against lawlessness and striving for the best we can do:
Romans 2:6-11
6 For he will repay everyone according to what that person has done: 7 eternal life to those WHO STRIVE FOR glory, honor, and immortality by patiently doing good; 8 but wrath and fury for those who in their selfish pride refuse to believe the truth and practice wickedness instead. 9 There will be suffering and anguish for every human being who practices doing evil, for Jews first and for Greeks as well. 10 But there will be glory, honor, and peace for everyone who practices doing good, initially for Jews but also for Greeks as well, 11 because God does not show partiality.