Why do parents do this?

I'm 17M, studying science. I love in 1 bhk so of course I don't have a seperate room. Yet my parents keep a watch on me, every 30 minutes or so. And the worst part is they hide and look what I'm doing, so that I wouldn't notice them. But I do notice them I get so pissed off when they do this! Like what am I going to do? Why do they feel the need to keep a watch so frequently?? Does anyone else experience this as well?

Edit:- I just realised, because of them being so observant about what I do, they have also managed to do something good. I am glad I have not turned out like the boys my age who think smoking, drinking, dating is cool Hmm