In 2011, in incidents of murder for which the relationships of murder victims and offenders were known, 54.3 percent were killed by someone they knew (acquaintance, neighbor, friend, boyfriend, etc.); 24.8 percent of victims were slain by family members.  The relationship of murder victims and offenders was unknown in 44.1 percent of murder and nonnegligent manslaughter incidents in 2011.

In short, people were killed by strangers 11.7% of the time. For 88.3% of murders (where the relationship between the killer and the victim was known), it was someone the victim knew.

The statistics are similar for sexual violence.

For juveniles and children who experience sexual violence, it is a family member of acquaintance 93% of the time. This percentage drops for adults to ~80%, i.e. in 80% of cases the victim will know the perpetrator.

https://www.rainn.org/statistics/perpetrators-sexual-violenc...

These crimes are very different from a mugging or robbery. It is easy to imagine why if you replay the situation in your mind — looking someone in the eyes as you stab them, up close takes a lot emotionally. It is a very intense situation. Almost everyone — if driven to it — will steal out of desperation and hunger, but very few can look people in the eye, walk up to them and stick a knife in them. There is a strong psychological aversion to hurting other human beings built into most of us and it's what keeps society mostly safe.