The comparison here does not really follow.
The MiG-21 was a famous, iconic and well used (by USSR) Soviet Fighter that also happened to be exported to other nations. It was designed and intended for Soviet use.
The F-5A on the other hand was designed and intended for use by allied nations and the export market. Not by the USA itself. The USA did have some F-5A for training purposes in order to teach foreign nation pilots how to use their F-5A, but America did not strictly use them at all. The small amount they did, were modified into the F-5C variant and later they had