No evidence of past or present life would be found anywhere in the solar system except on earth. The space program has at least shown there is no life anywhere else in the solar system. They therefore expected an abundance of life in Space.
Message theory says that such presumptions are mistaken.
The designer had a different goal in mind. The designer fashioned life to tell where life came from. That goal would be ruined by other intelligent life in the universe, since they might be mistaken for the designer. An observer might attribute earthly life to the wrong being. A biomessage sender would take steps to prevent that misinterpretation, and thus would not create intelligent life indiscriminately.