Its odd at first, but springs from economic principles, mainly sunk cost fallacy.

If you invest 2 days of work and did not find the root cause of a bug, then you have the human desire to keep investing more work, because you already invested so much work. At that point however its best to re-evaluate and do something different instead, because it might have a bigger impact.

Likelihood that after 2 days of not finding the problem, you wont find it after another 2 days is higher than starting over with another bug that on average you find the problem earlier.