`$_SERVER` doesn't hold server-originated data. It holds served-possesed data about the execution environment. This is, among other things, information about the request currently being processed.

So GATEWAY_INTERFACE, SERVER_ADDR, SERVER_SOFTWARE, and SERVER_ADMIN aren't server-originated data?

While things like HTTP_USER_AGENT and HTTP_REFERER are from the client.

I see a distinction between these. Server and client.
Some of these can be trusted, others can not.

It is the web server that communicates with the client, not PHP. It sets a number of variables about the current executing request. It is not PHP's job to try to figure out if a piece of information set by the server originated from the server or the client. In fact we have no way to determine this. We can make some guesses, but then when another server comes along and sets something we haven't seen before we won't know where that came from. We simply put everything the server sets into the $_SERVER array. This is simple, consistent, future-proof and easy for everyone to understand. What you are suggesting would lead to chaos and bugs if we were to try to guess as to the origin of every piece of data provided to us by the server.