Why is it considered sub-par to re-test something that has been shown before as opposed to addressing a knowledge gap?
In short, a PhD is about something that has not been done before. If there is no gap in the literature, there is no PhD, and defining the lack of knowledge is the first step towards covering it. A PhD is about novelty and exploration, not tweaking, and even PhDs that improve existing methodologies need to fulfil that requirement in some way by showing theoretical or technical expansions, or at the very least an application from field B that is novel in field A (even that for many people is not enough). A PhD explores ground already covered is pointless because it does not show creativity and originality of thought, which are fundamental for a researcher.

On the other hand, academic papers can (and frankly should much more) deal with reproducibility and testing/verifying existing results. There is a strong tendency in modern research to present positive results and novel findings, so papers that deal with what has not been found or not has not been replicated fall out of favour. That is an important problem, in my opinion, and should have been encouraged by the practices of conferences and journal editors/ referees, but belongs to a different discussion (even commentaries are under extinction).