As an avid motoryclist, who has motorcycled extensively in Europe, I don't think you have the reason.

I have seen a similar difference in how motorcycle riders are treated by other vehicles on the road (a lot better) compared to the US.

I don't think it has anything to do with politics. Were motorcyclists or bicyclists treated better when JFK was president, or when Bill Clinton was president? Nope. And was I treated better or worse in France when the liberals were in charge, versus the conservatives? Nope. And I have been treated a lot better in super conservative Utah, versus liberal Massachusetts. And treated a lot better in a liberal state like Minnesota, versus a more conservative Missouri.

I think the difference is that many/most car drivers in Europe were at one point motorcyclists (or riders of mopeds or scooters). The price of gas is so high, many start with two wheelers. So even though some eventually no longer ride motorcycles, they have a MUCH better understanding of how bikes operate, and are much friendlier to motorcyclists. This courtesy extends to bicycle riders. And their licensing is much more difficult to obtain than the US. Driving is considered more of a privilege, rather than a right. The same is true with bicycles. And look at the crowds that motorcycle racing in Europe draw versus the USA. Motorcycle race crowds there look more like a US NASCAR crowd.