Caller reports person sitting in parked gray sedan

Why do people always feel someone parked somewhere is suspicious? I used to work in home health and would drive a distance and sometimes arrive early and would park down the street. Some lady called the police. Nobody came to the car and I did not get out. I do it now as well because I drive for a company and sometimes I have to wait between people and appointments. We are just chilling out and lots of the time I check my emails or play a game. Nothing illegal about parking somewhere!!