Because she can. That's honestly the best answer I can give. She is a puppy and she has a mind of her own and she is young. She will misbehave, she will poop in places she shouldn't, she will drive you insane at times. BUT.....she will give you so much love and loyatly and entertainment that you will forgive her every transgression (at least, that's how it should be). Please be patient with her, it will take her time (months, if not years) to learn all the rules and even then, there will be times she doesn't obey them. She is like a child and you have to be patient and consistent.
Sometimes they have an area they like to do their business in. With Duke, it's the front room. No reason, that's just where he wants to go. About 4 months ago, we blocked the room off and there hasn't been a problem since. Sometimes it's better to take preventitive measures rather than try to figure out why.