PhD programmes usually require by definition that you discover "new" stuff. The difference between a bachelors/master's thesis and a PhD thesis is almost exactly that. Unfortunately reproducing doesn't tend to count, and even then if you reproduce a bunch of stuff just to find that it works, you haven't done anything novel.
In order to get a PhD out of it, you'd likely need to do a large number of reproducibility work, specifically going off some hypothesis like "method X has failings due to Y" and then testing it by doing the reproduction or similar. This makes it not low hanging fruit. In addition reproducing work is often very hard and time-consuming as you discover just how much is left out of papers/SI/appendicies.