Whenever they have, they have been bought by larger and richer (mostloy US) tech companies. Look at Deepmind, Skype, Nokia, Tandberg, etc... Arm is another (although Japanese rather than US-owned). There are also many cases where European founders base their companies out the States for access to higher funding (eg Stripe, Spotify). Another factor is that US multinationals have a large presence across Europe in terms of employment - if a large component of the top tech talent of Europe is employed by US companies then they are less likely to build large European companies.