A greedy algorithm (in the academic sense, although I suppose also in the colloquial sense) isn't the optimal solution to every problem. Sometimes doing the next most valuable thing at a given step can still lead you down a path where you're stuck at a local optimum, and the only way to get somewhere better is to do something that might not be the most valuable thing measured at the current moment only; fixing bugs is the exact type of thing that sometimes has a low initial return but can pay dividends down the line.