If the Dajjāl only appears at the end of time, why did every prophet warn about him?

The observation behind the question is a fair one. Why would Noah warn about someone who would appear thousands of years after his own community was gone?

The fact of it is established: “There is never a prophet who has not warned the Ummah of that one-eyed liar; behold he is one-eyed and your Lord is not one-eyed” (Sahih Muslim 2933).

Several things are being achieved by that universal warning:

It measures the danger. Nothing else is warned about by every single prophet. The repetition is itself the statement — this is described as the most severe trial to face humanity, and the emphasis communicates that in a way a single mention could not.

It arms every generation, not only the last. The pattern the Dajjāl represents — a compelling figure claiming divine authority, backed by apparent miracles, demanding belief — is not confined to one appearance at the end. Every generation has faced smaller versions of it, and a people warned about the greatest counterfeit are equipped to recognise lesser ones.

It fixes the test in advance. The detail quoted above is the whole method: he is one-eyed, and your Lord is not. The point is that the deception is defeated by knowing something about Allah beforehand, not by out-arguing the deceiver in the moment. That is knowledge every generation needs regardless of when he comes.

Nobody knows the timing. Since the Hour's time is known to Allah alone, every generation has had to treat it as possibly theirs. A warning delivered only to the final generation would arrive too late for everyone who wondered.

What this does not license is speculation about identifying him in current events, which the tradition does not encourage and which has an unbroken record of being wrong.