Because this messes up with some configure tests run at the start which link to a file that isn't universal, it seems.
Anyway, ENV.universal_binary can mess up with build system that know about universal binary, and don't use it every where for some reason.
I don't think it breaks utterly the build but it cripples the features enabled, because several tests fail where there shouldn't.
This method, allowing_UB and giving the right hint to compiler may also be applied to CMake I think.
In the main build (make -j *whatever*), qmake puts universal binary right so we only duplicate some flags that qmake used.