> why don't we just copy-paste it? ... Maybe license doesn't support it.
You did say the argument was bad, but a license that prevents you from making a copy manually but allows you to make a copy though the package manager isn't a thing, is it? In either case the output of your build process is a derived work that needs to comply with the license.
Unless, perhaps, you have a LGPL dependency that you include by dynamic linking (or the equivalent in JS – inclusion as a separate script rather than bundling it?) in a non-GPL application and make sure the end user is given the opportunity to replace with their own version as required by the license.
You did say the argument was bad, but a license that prevents you from making a copy manually but allows you to make a copy though the package manager isn't a thing, is it? In either case the output of your build process is a derived work that needs to comply with the license.
Unless, perhaps, you have a LGPL dependency that you include by dynamic linking (or the equivalent in JS – inclusion as a separate script rather than bundling it?) in a non-GPL application and make sure the end user is given the opportunity to replace with their own version as required by the license.