It’s driven by a fundamental shift in political economy. Technology centralizing power and making computation and communication (and thus control) cheap means that politics becomes a winner take all industry, in the parlance of the valley. Those who have read more than a little history understand what being the loser in such a contest means and thus the rush to the extremes and the escalation spiral (and of course the associated democratic backsliding) you see not just in the US but in most western countries.