Why does history rarely speak in terms of absolute truth?

Because history studies human actions, motives, and societies, which are often interpreted differently.

Examples:

Nearly all historians agree that:

the Thirteenth Amendment abolished slavery in law.

But historians disagree about questions such as:

What was Lincoln's primary motivation?

Was emancipation mainly a moral decision, a military strategy, a constitutional necessity, or an economic transformation?

The event is agreed upon; the interpretation is debated.

When I said "rarely," I meant that there are some propositions so well documented that they are effectively uncontested but most explanatory claims remain open to debate.