I think there are several reasons:
  1. There are differences in societies which might explain to some extent (higher taxation reduces incentives to found a start-up, cultures are less entrepreneur-friendly, etc.)
  2. There are nevertheless many tech companies and there is a lot of talent in this field. However in my opinion they are often driven by engineers ("techies") and not business people. To illustrate this with an example, think about Nokia which used to be world market leader in mobile phones. It was a great product designed by talented engineers, but then came Steve Jobs with Apple introduced smart phones and brought customer experience to a completely new level and Nokia's fall began. A similar pattern might be seem in the car industry where German car industry ("German engineers") are being overtaken by Tesla and Chines e-car industry.
  3. Lastly there are success stories of tech startups in Europe. However it seems that Europeans are happy to create a unicorn and exit (which is a huge achievement), but Americans aim even higher / Chinese do not sell their companies.