How would you propose finding more drugs for pregnant women? Because generally double blind clinical trials is the best way. Experimenting on pregnant women is considered unethical, which is part of the problem.
Yes, it would be great if we knew more but the fact of the matter is that there is neither the commercial impetus to study more, more the ethical ease of consent from a foetus.
Most data on safety comes from drugs already in use and 'presumed' safe due to their long standing use. Newer drugs are avoided but when taken without realizing they are pregnant or if there is no later active and studying the outcome is the only available options. That doesn't give much data.