1. No unified capital market and banking union, hence funding is restricted to per country Capital accumulation.
2. Still no unified digital products, payments, and IP market. The cost of selling any sort of IP in the EU is orders of magnitude higher than in the US.
3. Most countries have some sort of ridiculous immigration and work permit regulation, hindering startups to hire people easily and from anywhere.
At this point it is easier for somebody from the EU to incorporate remotely in Delaware, setup a bank account through Atlas Stripe, do fundraising in the US through Angellist, hire globally through any number of online platforms, and close deals with ready made YCombinator SAFE notes, and travel with a visa waiver every quarter cheaply to the US, than to even try incorporation in the EU.