Silicon Valley veteran here. Europe does tech, but we Europeans are far more focused on scientific solutions, fintech and sustainability, long term solutions for our continent and the world at large than creating overvalued startups like "Facebook" or "Twitter" whose only sole purpose is to go IPO and generate vast sums of cash for what amounts to little more than a pasttime.
We have lots of small startups and most of those are financed with the founders' own money, in stark contrast to venture capital in the States.
Silicon Valley has such a concentration because of climate and infrastructure. Climate is mild year round and the weather great. This, coupled with venture capital and plentiful job opportunities draws more qualified people.
Unfortunately only a handful of countries in Europe, like Spain and Portugal have that kind of weather, the rest is frigid temperatures during Winters and sweltering heat during Summers. Although Spain has started to pick up momentum in tech startups, it's still at the beginning and the economy there will require at least two more decades to pick up. Then there is Switzerland, a tech startup paradise, but startup companies there are focused on solving scientific problems and fintech.
One of the major differences between Silicon Valley and the European mentality is this: we don't have enough venture capitalists and those that we do have aren't the kind that would invest in companies like "Facebook" or ones that don't have a solid business model to begin with. On top of that, the founders don't want to just get rich overnight, they want their company to be sustainable at least until they're 65 and beyond, and they want the decision making power that a startup under venture capitalists wouldn't have.
In summary: a difference of mentality, and different goals, since the standard of living in Europe is much higher, focus is on different priorities.