It's mostly because of the VC Ponzi scheme. US investors invest in US incorporated companies because it is easier to sell them and to get other later stage institutional investors interested.
There's two ways how companies can quickly exit nowadays:
1. Acquihire - and the companies buying normally want colocated teams. Acquihires are predominantly happening because the 10x-ers can leverage their capacities by incorporating and effectively bringing forward their entire future earnings to the present through an acquisition. The companies like it because the can lock-in 10x-ers for 4-10 years, which usually only stay for 6-24 months in one company and jump ship very easily, taking company knowledge with them. They generally have to be incorporated and operating in the US as it is difficult to enforce such contracts internationally. This is also the fastest way for seed investors to get big multiples back without having to wade through all the subsequent rounds. As a seed investor you also don't need to pick a winning product or biz model, rather just a very smart team. The best of these brain-investors have contacts in all the big corps to advertize their investments to them well ahead of time.
2. IP is most often useless without the people and knowhow who can use and implement it. That generally means the people who developed it. Most loss of IP value doesn't come from simply Chinese copying, or even patent wars, rather the people developing and using the IP getting hired away to the competition. By having them all in the US or even State such as California, and keeping them on a 4-10 year contract, you firewall that IP from the competition.
Combine the above two with the much larger and more liquid unified financial market of the US, and you can bid your equity up due to demand pressure. The same company, with the same tech, and same users, and same development team, can be 5-20x less valuable in terms of equity if located in the EU. This is also why there are so few large exits in Europe as compared to the US.