Because attacking people for beliefs you don't like doesn't exactly make things better.
A lot of the people that seem to be trying to attack those they see as facist would be just as bad in the power as those they oppose. Except you know, with different targets.
The problem I see with a heck of a lot of online conversations is that it ridiculously fails this rule. Often times there are some issues on both sides of the coin worth discussing, but it's hard to discuss things rationally when the loudest voices on both sides are acting like spoiled toddlers or often worse. Often times, the extremists dominate the conversation and the resulting discussion is toxic.
Violence is never the solution, but isn't it possible to be anti-fascist (or against any other political movement that more relies on emotionalism instead of rationalism) without being violent? I would think so.
Responding to political concerns with violence just begets more violence. It just entrenches people's beliefs even further, inspires revenge from the 'other side' (or at least, their own axe crazy extremists) and eventually causes everything to erupt even further (through a civil war).
Honestly, I'd say this part in the most recent series of Doctor Who sums up some of the issues with people trying to solve all their problems with violence:
If those more violent parts didn't put the fear of God into the government, he could have been safely ignored.