Some civilizations may be so preoccupied with internal problems (population growth, environmental problems, weapons of mass destruction) that no resources are available for interstellar spacecraft (probably true for us for the next few centuries).

It may be that only very long-lived civilizations develop an interest in sending spacecraft throughout the Galaxy (and this may occur in waves of settling and expansion).

It is possible that some civilizations are so advanced that they have no interest in such a young civilization like ours.

Perhaps advanced civilizations have designated Earth as a "wildlife preserve", and they will only secretly monitor our condition until some future time ("Zoo Hypothesis"; cannot be tested in any effective way).