The reason you need a VPN to stream US networks in Europe is that those networks
can only be viewed by devices using a US-based IP address. It's a somewhat
arbitrary rule, not a technology issue.
It's a licensing issue. Streaming services are licensing their content from the rights holders, and that content is licensed to a particular service in a particular or multiple jurisdictions. A US streaming service that is not available outside the US is most likely because the service is not licensing content rights outside the US. Netflix's content in the US is going to be somewhat different than say the UK, depending on what is being licensed to who and where. In the US, to get British/European content I enjoy I have Britbox and Acorn, but in the UK those British shows I want are all on Netflix.
When using a VPN, you're working around licensing, and because of licensing streaming services' terms usually bar the use of VPNs to access their content. Netflix does block known VPN servers.