This is true of cis women transphobes as well. TERFs don't feel threatened by trans women. They know we're not physically dangerous, to them or anyone, really, and that they can harass, abuse, and direct violence at us with impunity.
They would prefer to push us back down into the societal underworld, back into gay subcultures or as powerless third gender sex objects for men.
Again, it's the assertion of our womanhood that angers them because we're claiming to be their equals.