The reason as to why your parents always volunteer you and your brother to babysit is because they want you to be more responsible. It could also be because they do not want you to spend your free time doing unconstructive things.
Most parents, in general do this to help their children "gain character" and become more responsible and mature. i can not say for your parents, as i do not personally know them. maybe you should ask them one day.
My mom MADE me and my brother watch our two younger brothers. We had to take them everywhere we went. Every single day, no matter what. It was so that we wouldn't do anything she wouldn't want us to, because they'd tell on us. Lol. I hated it when it was happening, but now looking back it wasn't so bad. I learned a lot and grew up a lot as well. You'll probably think the same thing later on as well.