If I understand your question correctly, you are asking why your example, "internet psychology", is not "internet's psychology".
Without having more context for your example it's hard to give you a clear answer.
I can see two possibilities.
The first is that the idea of possession is not present. "Internet" is an adjective adding additional information to "psychology" about what kind of psychology we are talking about. Similar examples would be course names at a university like "developmental psychology", "human psychology", or "military psychology".
If possession is involved, and again without more context it's impossible to say, then you have a very interesting point. English has two forms of expressing this kind of possession; the first would be the internet's psychology and the second would be the psychology of the internet.
Native speakers tend to distinguish between these two forms unconsciously and their choice is often influenced by the idea of animate and inanimate objects. English speakers tend to use the possessive 's with animate objects and the "of" form with inanimate objects.
Please note that this is a tendency and may be influenced by the usual dynamics of language usage (socio-economic, geographic, etc...)
If all this is not clear I suggest the short free video here.
I hope that helps,
author of The Secret Lives of English Verbs
Student or Learner