Signal, originally known as TextSecure, worked entirely over text messages when it first came out. It was borne from a different era and and securing communication data was the only immediate goal because at the time everything was basically viewable by anyone with enough admin rights on basically every platform. Signal helped popularize end-to-end encryption (E2EE) and dragged everyone else with them. Very few services at the time even advertised E2EE, private metadata or social graph privacy.

As they’ve improved the platform they continue to make incremental changes to enhance security. This is not a flaw, this is how progress is made.