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.

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.