Here's why:
* It represents an abstract concept: Violence is an idea, a force, or a behavior, not something you can physically count.
* It doesn't have a plural form: You wouldn't say "two violences." You would say "two acts of violence."
* It is often used with singular verbs: "Violence is a problem in our society."
However:
There are times when "violence" can be used in a countable way, usually when referring to specific acts or instances of violence.
For example:
* "There were several acts of violence at the protest."
* "The police are investigating the latest incident of violence."
In these cases, "violence" is used in a more concrete way, referring to specific events.