Because in general, it can be quite normal to not free memory as soon as possible. Garbage collection can get off its lazy ass and get to work whenever it feels it's appropriate. Surely you have a trash can inside your house, right? Do you take it out as soon as you put one thing into it? Or do you wait, put more and more things into it, and only take it out when it's (almost) full or you for some reason feel like it (like you having nothing better to do)? So unless it's actually a problem, like you getting full-memory errors, everything might just be going according to plan.
That said, if I'm wrong and misjudge the technology and such increasing memory consumption likely does indicate a real problem, please let me know.