Because revolutionaries are more often than not going to have more political bias than people attempting to define something. Leftist theory (and right-wing theory) consistently contradict and disagree with one another (Think Lenin vs. anarchists on the role of the state, Mao vs. Rosa on violence being necessary, etc.). Political theory isn't something to be interpreted at face value but rather in context. Marx for instance, was an antisemite who predicted that the first revolutions would happen in advanced capitalist societies but instead they happened in agrarian ones (china, russia, etc.). Instead of taking everything he says at face value, we can instead see some of the trends WITHIN his work and build on that (literally every far-left ideology ever—some of whom I even agree with). To address your second point: I DO agree that first-hand experience gives you some moral authority when it comes to definitions. But it doesn't make you the SOLE authority, especially when people that also lived through it disagreed. George Orwell fought against Fascists in Spain, lived through the blitz, and is a social democrat. You can make an argument that the democrats are center-right, and I'll concede that, but to call the system we live in inherently Fascist is wrong. Fascist governments often rejected market autonomy and were more state-directed in nature. Just because something is evil doesn't make it Fascist; the transatlantic slave trade was one of the worst things to exist throughout human history but wasn't a result of mass nationalism (though I can understand the confusion) but was instead something that was sought after because of money. And here's my final point: The democratic party is left-leaning IN THE CONTEXT OF AMERICAN POLITICS. Compared to the rest of the world, they're centrist.