Why are proxy settings not always respected by programs launched from Term?

Why do some programs evade the proxy settings at Settings > Wifi > Details > Proxies? I would expect that a system-wide setting would apply to all traffic. However, I can set the HTTP and HTTPS proxy to a local IP and port, use an off-the-shelf proxy (like Charles), and kick off certain commands that do not appear in Charles. A really easy one is curl. If you setup an HTTP proxy and then pop into term and do something like

curl https://www.google.com 

You will find that traffic does not go through Charles! Instead, you have to explicitly tell curl where the proxy is, e.g.:

ALL_PROXY=http://localhost:8888 curl https://www.google.com 

I'm curious what's happening here. Specific questions: 1. Is there any information about the type of traffic that actually routes through the wifi proxy settings versus traffic that does not? 2. Is there a way to route all traffic through a local proxy? Thanks for reading.