They are stressed
When a person gets stressed they might hate to hear a phone ringing because this might add to their stress. In such a case, the person might keep their phones silent to reduce the stress they are experiencing.
They hate phone calls
Some people hate phone calls. Those people might always keep their phones silent to avoid answering a phone call without feeling guilty about it.
They prefer texting
Some people prefer texting over voice calls. Those people keep their phones silent all the time to force people to text them instead of calling them.
They are always busy
People who are always busy usually hate phone calls because they distract them. Those people might always keep their phones silent to prevent wasting time.
They don’t want to be distracted
Some people always keep their phones silent to prevent distractions. Those people might want to enjoy their daily activities without getting distracted by phone calls.
They don’t like ringing sounds
Some people don’t like the sounds of ringing phones no matter what the ring tone is. Those people get stressed and anxious when they hear a phone ringing and so they keep their phones silent. (See Why do some people reply late to text messages?)
They want to forget about work
Some people keep their phones silent all the time to not get phone calls that would remind them of their work. Those people might want to enjoy their lives without being reminded of their work.
To feel more free
Some people experience more freedom when they keep their phones silent. Those people might keep their phones silent all the time to experience this freedom.
To become more productive
Some people keep their phones silent all the time in order to become productive. People who value productivity so much are very likely to keep their phones silent most of the time. (See Why some people don’t like to talk?)