Learn English – Is “Unless you had told me about Sue’s hair, I wouldn’t have noticed it” correct and natural-sounding

Because that is the form we use in English for counterfactuals. Historically it was the past subjunctive, but in Modern English that is distinct from the ordinary past in precisely one case: I/he/she were rather than was. This is why some people say

If I were to tell you, ...

(Others have lost this and say If I was to tell you ...)

Teachers of English as a foreign language refer to this as the second conditional (a phrase I had never heard until I started reading this site).

What if I tell you ...

is the so called first conditional, and refers to an unknown future, rather than a counterfactual. With a first-person subject, there's not too much difference in meaning, but if I tell you is saying it is quite possible that I will tell you, whereas if I told you is considering it as a hypothesis, probably less likely.

See Second conditional in Wikipedia for more information.

The first statement,

If I were you, I would look for another job.

suggests that the advised action is something that the person to whom the sentence is addressed can perform to beneficial effect right now.

The second statement,

If I were you, I would have looked for another job.

implies that the action would have been appropriate in the past, but that there is no guarantee that doing it now is potentially beneficial or even possible.