There are at least two ways to argue this.
First, there is the moral/ethical argument. When people use their talents and their efforts to make something, it belongs to them. To be ethical, we must reward them for their efforts. We should not simply take things that came out of their efforts.
Second, there is the practical argument. If we do not protect intellectual property, no intellectual property will be created. If the people at Microsoft do not think that they will be adequately compensated for their work, they will not create the sorts of software that have made our lives so much easier. If musicians do not think they will be compensated, they will not create the music that entertains us.
Thus, we can argue for copyright protection both on ethical grounds and on practical grounds.