Likely because if the effect of "bad" code is positive enough, you have incentive to just let it keep on being "bad" and not touching it, so you can keep reaping the benefits.

Also, if you're on a tight timeline for a critical function, you may want to push ugly/inelegant/dense/"bad" code just to get the function in prod a day sooner. This doesn't mean the code isn't "bad," it just means that the business needs outweigh the ugliness.