Because in the realm of religion, people find it very easy to say "I can't" because they have been trained to believe it - or, worse, they've been trained that it's inevitable. This will manifest itself in many ways, depending on the tradition of Christendom with which they are affiliated.
Some will say, "I can't do Good because God hasn't given me the ABILITY to do Good. Yet.'
Others will say, "I can't - and shouldn't - do Good because I don't need to. I'm already saved by my faith alone."
And most pernicious of all, some say, "I can't be perfect or holy because I'm only human, and I'm unable to do Good because of Adam's sin."
Let's look at these claims one by one. To say, "I can't do Good because God hasn't yet given me the ability to do Good" is effectively putting the ball back in God's court, laying the blame at God’s feet for failing to provide strength for us to act, and copping out of our duties and responsibilities.
While it's true that God will help us on our journey, and embraces us in our darkest moments, to make God do all the Work He has instructed us to do makes Him our slave, not our God.
It's also an amazing slap in the face to God to claim that we are unfit and unable to obey His moral commandments. God, who has freely given us the gifts of senses, strong bodies, great minds, and tender hearts, calls us to use them in His service, and to serve Others in His name. We live on a planet with amazing resources with which to do the Good Works God commands us to do.
We have the knowledge of what "Good" means to God, through the books of Scripture, which reveal His Prophets' teachings and warnings, including the teachings and warnings of His adopted Son, Jesus, whom He sent out to be a PERFECT example for us to follow. Friends, "we have no ability"? We have no such excuse.
Every means of improvement and happiness is the free gift of God, but then the improvement itself depends on ourselves, and from our actions alone will we be judged.
Others, mainly in the Protestant tradition, have learned a twisted version of "salvation" by learning to rehearse the line, "I can't - and shouldn't - do Good because I don't need to. I'm already saved by my faith alone." This is the "Wide Gate" version of Salvation, and is a gross misunderstanding of the words of Jesus - God's Anointed Prophet - who brings us Salvation FROM SIN as a free, initial gift, but then calls on us to earn our eternal salvation through obedience to God's Will through our Deeds, by which we will be judged and rewarded in Heaven.
And, when specifically asked "what must I do to inherit eternal life?" Jesus replied that we should obey the commandments (Mark 19:17) and said that mere faith alone would not be sufficient (Matt 7:21).
Next, to claim, as some do, that the Moral Law God wishes us to follow is too hard is false. In the Book of Deuteronomy, we learn that God's burdens are "not too hard", and that we "CAN do it", and have no excuses for disobedience (Deut. 30:11-15.) Jesus teaches that the "Yoke" which is represented in his teachings is "light" and easy to handle. To plead that it's too hard to do Good is to spit in God's face and deny His Prophets and the Scriptures.
Also, to fail to separate the RITUAL Laws - which no Jew even follows now, after the destruction of the Temple in Jerusalem in AD 70 - from the MORAL Laws which are always in force, forever - is willful ignorance. The vain traditions of man which Jesus condemned, such as the ritual hand-washings that the Pharisees taught in Jesus' day, are also not binding on us. Those who find the Ten Commandments "too hard" to follow, and therefore will be judged rather harshly, if God's pronouncements are true.
Finally, to saddle Adam with our excuses ("I can't be perfect or holy because I'm only human, and I'm unable to do Good because of Adam's sin.") is not only wrong, it's also unbiblical - even if it is a popular excuse today.
In the Biblical stories of Genesis, Adam's sin, and Eve's, was supposed to have made women have painful births and to have made us labor in the fields rather than living in ease in the Garden of Eden. But any thought of using this story to allow us to blame God for our sinful behavior (because He must enable us to do Good Work for us, "lest we boast") or to excuse us from acting Righteously because we inherited a "stain" of sin and thus could not obey God, is unjustified. And unjust.
God, through Moses, said that His Law was “not too hard” so that we “could do it.” (Deut. 30:14) And God expected us to obey his moral commands, as Jesus repeated consistently. He said that no one else would be charged with our disobedience, and no other would be responsible for our actions except ourselves.
God told Adam's son, Cain (just one generation after Adam!) that he COULD avoid sin, and MUST do so (Gen. 4:7.) If he was able to freely choose to do Good, so must we! Some also see Sin as a disease infecting us from birth. If sin is a disease we are born with, it is not our fault and cannot be blamed. In that case, any Good we do is incidental, and certainly cannot be a basis for judgment, since it's not our fault. But that's just men spinning excuses.
Further, the idea that poor humans are unable to obey God's commands is contrary to every line of Scripture, through which God and those Whom He sent repeatedly tell us to "choose" and "obey" and that we will be judged according to our choices.
In fact, Sins are ACTS that are done, not a disease we inherit, and they can be undone when we go to God in humility, ask His forgiveness and ask that all our previous Sins be covered over and forgiven by Him. From that point, we must commit our lives to obedience to His Son, Jesus, the Anointed Prophet of God and submit to humbly walk with him, relying, as he taught, on God's grace and forgiveness and growing into the Righteous Perfection God knows we are capable of achieving.