Say... for an example that resembles the OP project... In a company I work for, we have a FUSE-based (GNU/Linux) filesystem that we use to access Azure Blob Storage like any other mounted network share. A puny tiny project that's a fork of someone else's fork of someone's project, all FOSS (Apache2-licensed, though, not *GPL, but I believe it's irrelevant here), but hope it should still suffice as a quick example. There's literally no reason for us to keep it secret - we're end-user ourselves, enjoying cp and rsync working nicely. Well, maybe not completely nicely - I'm aware there are still some issues with that filesystem, but at least, with the patches, it works OK for us. So, sure, we've put all our changes on GitHub. And, I'm sure, every our developer who had touched the code to solve the problems we had with it got paid for their time. :)
So, I guess, it's every time the engineer is an employee of a company that's an end-user themselves. I think the only case where GPL/AGPL doesn't fit is when you license the software to the end-users to run, and you don't want them to re-distribute and modify software on their own but rely on you for all their needs. And it seems that original project author doesn't fancy this particular business model.