1. Fix the relatively low tech wages [in Europe]
You can make enough money in Europe to have enough runway to start a company. Especially if you give up working 40 hour weeks and having 6 weeks vacation.
2. Taxes [are high]
Many places in Europe have lower taxes than the US. Startups rarely feel the effects of taxes since they spend all their money on growth. To the extent they do it's just another cost.
3. The anti-startup culture [is bad]
In practical terms European welfare states are far more accepting of failure than other parts of the world.
4. Severe lack of capital
This can be a concrete problem, but most people making this statement have not saved their money and vacation time for at least year, taken leave from their job and started a company. So why should investors invest in them?