The message never acknowledged can keep occupying the sender long after the reader moves on.

Silence is an empty container and the sender fills it. The reader glanced at it, meant to answer properly, and got carried into the rest of the week with no notion that the gap was being read as anything. Meanwhile the sender has an open loop, rereading their own words and drafting explanations. Unanswered things stay open on one side only.

If something has gone unanswered and it matters to you, ask directly rather than deducing. A short second message is not a demand. When you are the reader, send the two-word acknowledgement now instead of the considered reply you are not going to write.