Most religions and their sacred documents acknowledge the existence of Angels (good).  Most religions and their sacred documents also acknowledge the existence of evil spirits (sometimes called fallen Angels).  In some religious documents, Angels defend people from the work of evil spirits… but only up to a point.  Most religions acknowledge that humans have been given ‘free will’ to make their own decisions.  ‘Free will’ means that humans have been given the freedom to make their own decisions, but they must face the outcomes of those decisions. 

It is extremely rare for Angels to show themselves, because they respect ‘free will.’  If they regularly showed themselves or communicated with humans, they would be interfering with ‘free will.’  The existence of Angels may be one reason why evil spirits are not doing more physical harm.  In some instances, there may be Angelic forces stopping them from doing so.  But as people continue to engage evil spirits, the protective forces may begin to step back.  After all, the human is demonstrating that they want to participate (free will).