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?
If you look at taxes that both countries have that can seem true in some places, but that's because European countries tend to have VATs and the US doesn't, so the rates on the shared taxes are often lower in Europe (also, the US tends to have more tax exceptions, so that the actual tax effective tax rates are lower than the nominal rates.) The US has the third lowest overall tax burden in the OECD, with only Chile and Mexico lower.
What are the best prospects for your friends who did attend university or anyone else who didn't work 50 hour weeks for 5 years and become a partner at a consultancy company?