Left and right are always relative to a centre, and different in every country.

Or, we can look at the spectrum of politics that exists and, observe that there are political groups outside of the Democrats and Republicans even within the United States, and see that you're wrong.

Only a proportion of the full political spectrum is viable.

For example, we know pretty conclusively at this polint that communism doesn't work, which is why the "left" these days is social policies overlaid on a free market system.