People who use GPLv3 want the code to stay open/libre under any circumstances. If this is the goal, why not use the AGPL instead, even for applications which are not served over a network?
This takes away the possibility that people integrate parts of your program into a proprietary network application, even if this seems improbable. There’s nothing to loose with using this license, but potentially some gain.
Only reason I can think of is that AGPL is less known and trusted which may harm adoption.
Oh, most companies will pay someone to maintain an internal fork, but hardly any will contribute back. Sometimes that’s due to lazyness, sometimes it is the idea that nobody will care for the company internal stuff, but most of the time it is outright forbidden to share internal IP even when that comes in the form of patches to open source code.
In my experience it is safe to just ignore that case and not care about corporate convenience when starting any open source project.