Equal parts "non-zero threat of physical escalation" and "empathy triggered by proximity to another human."

It's hard to stare into someone else's eyes and feel... nothing.

It's pretty easy to look at text on the screen and forget the person on the other end is like you.

it is lack (or significantly lower risk) of pretty much any threat. In real life people with different opinions are punished in many ways, not just physical (is anything i'd say the pure physical punishment is frequently not the main concern). Being anonymous online (or hard to reach in some other way, like say residing abroad) allows at least some freedom of speech.