Because software development is not the 100 metres. You could spend weeks on a particularly elusive and obscure bug and change one line of code. While you were doing that, your colleague built 2 new 'oh wow' features, banging out hundreds of lines. There is no standardised measurement for 'performance' that can rank those two achievements objectively, so to assess performance, it comes down to the subjective opinion, motivations and political goals of the 'assessor'.

More to the point, if the subjective performance metric rewards the latter and punishes the former, the net result is more features AND more bugs.

In other words, it's crap wherever it occurs.