Not given back to the original authors, but given to the users.

Only if you give those users your modified version.

If I host GPL software on a webserver and my users use that webserver, I don’t have to give them the source code for modified GPL programs. This is fairly common.

Depends on which GPL you’re using.

Issue is that many large corporations use FLOSS internally with heavy extensions/modifications and never give back to the community. They don't have to, since all users are in-housr, and those tend to have access to the source code.

But that's ok. If upstream is somewhat active then it's just a pain to keep maintaining your in-house patches, compared to sending them upstream. So that's automatically a motivator. If upstream is not active, then it does not matter anyway.