I've read all the comments and looks like no one has mentioned this, but as someone who worked a total of ~25 years, on both continents (10+15 years), I have the following observation, and it really is a simple one:

American hardware/software engineers work really, really hard, compared to their European counterparts.

Given identical goals, same level of funding, same underlying tech, and similar market size, a US startup will almost always be more successful then its European counterpart, simply because of the difference of total amount of work done in a given month/year.

I've worked with outstanding people in both continents. Top-notch, impressive engineers. I can easily say, the output of my US colleagues were significantly higher.