What good is a VPN if Hulu and Netflix still know where I am?

You mean you change your IP to a US IP to watch shows/movies in a non-US country? I think Hulu & Netflix can detect VPNs and restrict access. I just use a SOCKS5 proxy with a high reputation score.

VPNs were originally intended for networking multiple devices under one subnet. So... that.

Can anyone explain to me how they can detect a VPN?

For hiding your network traffic from your ISP