Taking a global view, I’m not convinced it’s a case of pendulum swings, as insanity has cropped up in places that were far more multipolar or where the trend has been mostly in one direction without any swings. The lunacy isn’t just in the UK and US, there are also populist fascists in Brazil, Russia, and the Phillipines, and populist wannabe-fascists in Hungary, Italy, et al.
I think the reason is that there are no consequences to anything anymore. Well into the second decades of wars in Iraq and Afghanistan, there’s been no action on war crimes, torture, and imprisonment without trial in Guantanamo, just for examples (indeed, instead there is still pushback on even investigating some of these things). A decade ago there was discussion about to what degree the West is now responsible for “fixing” things in Iraq, which now appears to have ended. Before, the West at least showed public shame and held hearings regarding involvement in torture and extraordinary rendition, without real action being taken; now there’s not even any display of shame about shameful actions. Why should there be? The Saudi government has murdered a journalist and carries on a brutal war in Yemen, and there are no consequences. Turkey, oppression of journalists and Kurds, nothing.
Russia continues to have troops in Ukraine (and Georgia? Not even sure anymore), and it’s barely discussed. Duterte promotes the murder of his citizens, Bolsonaro is publicly opposed to a majority of his population (by denigrating women, the non-white, and non-heterosexuals), and both continue in power by popular vote.
Of course there’s an economic factor to this too. Amazon, Apple, Google, and more pay essentially nothing in taxes, yet with their business being movable, countries fight to give them the best tax breaks, because getting 0.1% of their fortunes in taxes is still better than 0%. Banks caused a recession but it’s the taxpayers who pay for it by bailing them out while getting nothing in exchange apart from increased bank fees.
Since there are no consequences, there’s no reason for anyone to promote sensible long-term policies. Sensible government, and socially conscious long-term business policy, is too complex in modern systems to gain attention, while the other side always has solutions that are simple, easy to grasp, and also stupid and wrong. “A sensible environmental policy would involve a gradual transition to renewable energy and nuclear power at the cost of the coal, oil, and shipping industries which cause a majority of our pollution, with an interim period of either higher taxes or higher energy costs for all citizens” is a mouthful - “recycle and buy an electric car if you feel you must” isn’t. Or, for example, “Britain’s relationship with its European partners is a complex web of treaties and hard-fought multilateral economic benefits that are not easily unravelled, and were we to do so, it would take time and carry enormous costs in the short and long term, so taking other measures to safeguard our sovereignty while maintaining our European relationships would be preferable to a summary exit to an unclear future” is boring and seems vague. “Strong and stable, brexit means brexit” is memorable, and sounds decisive despite being meaningless. “With decreasing first world birth rates and aging populations, immigration is a net positive and steps should be taken to maintain and even promote legal immigration of those willing and able to contribute to our society” is wonky and uninteresting. “Build a wall” is idiotic, ineffective, and easy to remember.
At the end of the day, I think the insanity we see in populations’ votes is from a mixture of factions ending up voting for the greater evil, for different and even opposing reasons. You have the well-meaning misinformed (e.g. those in Norway calling to ban diesel cars outright because they pollute - what about biodiesel, which is carbon-neutral?), the willfully misinformed (save our NHS, Britain out of the EU!), the outright malicious (build the wall! No migrants into Hungary! Death to drug addicts!), the economically dissatisfied (climate change is a hoax, protect my coal mining job!), and the angry who just vote for any anti-establishment candidate (why have there been no consequences to those who lied us into wars in the Middle East, the bankers who crashed our economies, or the Brazilian politicians who bribed a majority of their parliament on a monthly basis using public funds?). In the “good old days” even pretty bad leaders seemed to intend the best for their countries most of the time and tried to do good work, even if it just meant keeping a quietly competent and effective corps of public servants running. Now there’s always a gleefully incompetent halfwit willing to spread comforting lies and bullshit for personal fame, likely getting paid by business interests along the way. As always, it’s the population that suffers.
As a postscript, a handful of companies and businessmen have apparently already pledged 600 million Euros to rebuild the Cathedral of Notre Dame, despite the Catholic church being one of the richest organisations on the planet. When it comes to receiving public glory for restoring a building, there is money. When it comes to paying taxes for the health, schooling, housing, and feeding of their fellow citizens, instead it’s the plebs who must tighten their belts.