The reason is very simple, if somewhat disheartening. Take a look at some of the literature on human behavior, particularly the studies on the "banality of evil" (texbook scenarios are the
Milgram Experiment and the infamous
Stanford Prison Experiment).
The sad truth pointed out by both of those studies is that approximately 60% of us -- all of us, even those of us who claim to be, and act like, normal ethical people in polite society -- will commit acts of cruelty upon another human being, even to the point of delivering potentially lethal electrical shocks to someone obviously in distress, if the social sanctions against it are removed.And those were both cases in which the victims had voices and (in the latter case) faces by which the perpetrators could witness the suffering they were causing.
In short, the majority of people will be cruel, spiteful bullies if they believe they can get away with it. For me, a good example is (oddly) watching how people treat pigeons (??): they're harmless, no more dirty than, say, hoboes, and live around us. But they are negatively viewed as carriers of disease ("rats of the skies" is such a cliché, and what's so bad about rats, anyway?), and most people wouldn't think twice about trying to scare them and threaten to cause them harm. It seems a bit melodramatic, but I often wonder why a person would
want to be mean to some random harmless animal. I think, sadly, that it's because most people
like being mean, and just need a venue to get away with it.
The Pinochet regime in Chile figured this out pretty quickly: you don't need to
make people commit acts of cruelty against their will. All you have to do is provide a venue for cruelty without consequences, and the people will come out of the woodwork of their own accord. And Facebook/YouTube/your local news station's comments section are just such venues.