If Allah forgives all sin, why should I do any good deeds?

I am asking this question not from a personal point of view, but more philosophically. I often come across Muslims who are openly committing every type of sin, and they justify it by saying, "only god can judge me and god will forgive me." I know god is the most merciful and forgiving, however, this seems unfair to those muslims who strive to do good and be good in their lives. I understand that between sins committed against other humans cannot be forgiven by God, and I am not referring to those. I mean sins such as alcohol, zina, shirk, etc. If all we have to do is repent and god will forgive, does this really mean we can live a life full of all sorts of sins, and repent shortly before death, and all will be forgiven ? What is then the incentive to live piously ?