Assuming your premise is correct: What did you expect?

The origin of modern America dates back about 400 years, the origin of Europe about 2000 years. Also, the reason for many early US immigrants was to escape ancient European social structures. Europe was already crowded before America was explored: there simply was less room for outside influence from other parts of the world.

Even so, there are also quite a few attitudes, customs, social structures, and institutions in the US that would seem out of place in (most of) Europe. In fact, the sentence may even hold true for many comparisons within Europe.

I doubt, one could conclude that -- due to such differences -- one country or nation is more liberal then the other.