The “no real single market” tends to not apply to software companies, which can spread to other countries in europe relatively easily. There are also some reasons why people think its unlikely to be a lack of capital. For example if there were less venture capital in europe, then that capital would go to only the most promising startups (VCs can pick and choose who they fund). So you would expect the return on investment to be higher than in the US. But that’s not the case.

Other reasons that impact scaling of startups are lack of know-how since highly skilled people tend to move to where the biggest companies are, e.g. silicon valley. And, and this one is always hard to hear, labor protections. It is a lot more risky to scale up when the people you hire are hard to let go if you encounter hurdles while scaling up. The eu countries with comparably many unicorns, are ones that have very flexible hiring and firing.