Lou,
Some of it may be due to the natural tendency of human beings to become more intelligent, more knowledgable, and more ethical over time
Although this is such a painfully slow process it is hard to believe it is happening at times, if you take the longer view of history it is clear that human beings were dumber and more nasty in the past. To take the US as an example, you've gone from slavery to lynchings to segregation to a black President. In the long view of history, it's certainly progress.
Anyway, so what I'm saying is that as society becomes more intelligent and ethical it naturally becomes more left wing. Of course, as that happens, the goalposts shifts and what was considered left wing becomes a centrist policy. But the centrist policies of today are the uber radical left wing policies of some years ago, and the Warren/Sanders/Corbyn ideas of today may also become the cross party consensus somewhere down the road.
There are so many examples of it.
Gay marriage: 1990s not legal in any country. Right wing vs left wing issue. Even Obama wouldn't speak in favour of it at first. Result: left wing policy becomes consensus.
Climate change: right wing denial lost, left wing won (at least in accepting the existing of science, if not in actually reacting to it much).
Minimum wage: In the UK, the right wing Tories used to be against any minimum wage at all as recently as perhaps the 1990s or even early 2000s I think. Now you will struggle to find any major politician in the UK that would speak against it. The Tories have actually been increasing it.
NHS: In the beginning in the UK, just the very idea of a public health service was a left wing idea. Now the Tories fall over themselves to say how much they admire NHS workers while hoping everyone forgets that it took a lefty government to ever create it in the first place.
Wars in the middle East: Take the Iraq war that was heavily opposed by the left wing side. Can you see a war like that happening today, supported by the UK. I think not. Nowadays the bar for western involvement in middle East wars is higher.
Then there's racism in the US. Now every company and person is tweeting that black lives matter, but a while ago it was only left wing people (as well as black people themselves of course) who really saw it as a major issue.
Now the left is also starting to win the broader war on the excesses of neoliberal capitalism, this decade will see change.
In my view, the "radical", progressive liberal left is missing a strategic opportunity to say "We are usually right!" When will people look at the historical record when they vote!
That being said, I am respectful of conservative ideology and think the right wing is more correct than the left on a minority of issues but there is also sometimes a pattern where the left stands for positive change and the right for resistance to change.
But in general this whole thing of Biden shifting to the left may just be the politicians following the shifting sands beneath their feet so that they don't fall over. Half the things Biden said in the 1970s or 1980s would probably count as right wing today.
