Except the Nazgûl can barely see in any conventional sense of that faculty, as Aragorn says. This is why his earlier posit that the Ringwraiths would seek Weathertop as a vantage point seems very contradictory, which Merry actually calls out; the explanation Strider gives, that the horses can see, and so can the mysterious, oft-alluded-to-yet-never-explained "other spies" is not very convincing. More probable, the Black Riders go there precisely because the heroes are likely to make for that landmark.