Yes, I do understand that people can take the BSD/MIT codebase and make a proprietary/closed source version. In that case you don't even see how the bug fixes a

Yes, I do understand that people can take the BSD/MIT codebase and make a proprietary/closed source version. In that case you don't even see how the bug fixes are implemented in the new closed source version. So you are free to independently come up with the most obvious, efficient fix. My point is that with GPL, the implementation of the fixes/new features is right in front of you, you see it, yet you can't merge it into your GPL-incompatible codebase. There BSD/MIT licenses don't prevent you from doing this either, though it's possible in practice it happens less. You can create not only closed-source proprietary forks of a BSD/MIT code base, but "source-available" or "shared source" proprietary forks, where the source is public but under a proprietary license that prohibits unauthorized redistribution or derivative works. There's nothing in the BSD/MIT license that requires you to keep your proprietary forks secret.