Why would a writer leave an assumption tacit or implicit?

A writer leaves an assumption tacit or implicit.

Explanation:

The writer could leave an assumption tacit or implicit. The reason might lie down in the writer's intention to give space to readers to figure out why the assumption of this specific is implicit.

The writer might want readers to read more carefully and discover why some characters or why some actions are happening. Also, some assumption is a trigger for some actions or motivation for characters.

If a writer doesn't write a novel or fictional book, the assumption might be a sign for readers to figure out or conclude some experiments or natural law or biological order in nature. The assumption makes the reader thinks about it and tries to find a solution for it.