Getting back on topic and answering the original question:
It's because they don't realize it is hate.
They view their derisive attitudes as a mere statement of fact from their perceived moral high-ground at best and mocking derision at worst, failing to realize that mocking derision is, in and of itself, an expression of hate.
And since they tend to truly not understand the definition of the word tolerance, they view and express their views to anyone with differing views with nothing less than mocking derision. Anyone who deigns to express differing opinions on anything that matters to them is, at best, an ignorant redneck if not a troglodytic neanderthal. And since they view even a willingness to discuss and consider alternate viewpoints as a form of condonation, they refuse to even participate in logical discussions - making them as dogmatic, illogical, ignorant, and backward as those they profess to be above.
Of course, the far right exhibits identical behavior - only with different talking points.
It used to be - or at least seemed to be - that this type of behavior was limited to a small minority of those who fell, in American political terms, at the extremes of the left and right viewpoints. These days it seems like this behavior is becoming much more the norm - even among those who profess to not possess more extreme viewpoints.
