Also, European culture is slightly different: most people consider a startup failure as something bad that you drag along for the rest of your career.
I've talked to some European VCs, and while they have big ambitions and are starting to gain momentum, it might take a while before the shift in the European mentality happens, and we end up with a thriving ecosystem. Some hubs (Switzerland and Berlin for example) seem to be doing quite good.
I also assume a big difference with the US is the fragmented market: different languages, cultural differences, legislative constraints, market demands, ... For example, in my experience high touch B2B sales are highly impacted by the language gap, in fact some countries (France, Spain, Italy, ...) are notoriously against working with people who don't speak their mother tongue. (On the bright side most of us Belgians do speak at least 3 languages, so it is still somewhat doable.)
As an alternative to VCs in Europe, you could look at some of the national and European subsidized tracks, although they tend to be mostly coupled to the trend-of-the-decade (sustainability is hot right now), and also require quite some effort to enter with no guarantees of success. (Disclaimer: it's on my to-do list, so no real experience with this.)
I would presume the combination of all of these factors results in people looking on other continents for better opportunities.
(I personally think Africa might offer huge opportunities in the coming decades, but that's for another time.)