[bunderbunder]

Because it's in production and people are using it.

I've run into more examples of broken code for which the above statement is true than I care to remember.

My original response was meant to be a rhetorical question, with the purpose of hinting that techniques for verifying the correctness of code might have existed existed prior to the 1990s. But "we haven't heard any complaints from users yet," while certainly a popular technique, is only slightly more reliable than prayer as a QA policy and I personally wouldn't recommend relying on it.

[Splognosticus]

That's kind of the angle I was coming from as well. In order to effectively rewrite something you first need to know what the existing code actually does. If you don't know that yet, then figuring it out is usually a lot harder than fixing whatever problems it has.

It's not that you should never rewrite, but it's always a lot more work than you think it's going to be.