How does low engagement lead to emails going to spam?

Low engagement tells mailbox providers that the sender’s content is irrelevant or unwanted. When a pattern of low opens, low clicks, and silent deletions accumulates, the filtering model begins to predict that future messages from this sender are likely to be ignored.

To protect inbox quality, the provider shifts the sender toward the spam folder where fewer users will encounter them.

It is the equivalent of a port authority noticing that no one ever unloads your cargo. Eventually they stop giving you prime docking space and move you to the outskirts.