Loosing followers.

I know, random topic, but, it really bugs me! For instance, on Tumblr I usually have around 720 followers. If I don't post for a day or two, or simply go to college for a few hours, it goes down to only 700 followers! The one problem with this that I can't quite grasp is the fact, if I'm not posting on Tumblr, how are they finding my page? I probably will never fully understand why people must scroll through Tumblr, don't see me posting something and therefore go onto my page and unfollow me! Why is there such a need? I don't have to post 24/7.