The issue is that install-lib requires building everything, which is a fairly long process, while install-dev only requires building headers and import libs, which can be done quickly (relatively speaking). It is quite convenient for the purpose of building a Wine-based toolchain. And if we want the toolchain to be useful, now that wine/unixlib.h is installed, ntdll.so needs to be available too for linking.
In Proton we used that to build third-party modules in parallel to Wine, which makes rebuilding much faster as they don't need to wait for a full Wine rebuild and install. It was possible before with the `dlls/ntdll/install-lib` `dlls/win32u/install-lib` targets, but they are gone now and there's no alternative to install only these two unixlibs.