Why should they? Are they going to make more money if they fix it? Of course not; people aren't basing their usage of Windows on the performance of the built-in unzipper. Advanced users just install a 3rd-party tool, the clueless masses just use the built-in thing and wait and think it's normal. Companies like Microsoft don't care about building the best product they can; they only care about maximizing profits, so the solution that's "good enough" is what they go with. What they have works, poorly, and costs them nothing to maintain, so that's what they stick with.