In at least some cases, this is pretty clear cut. Don't roll your own crypto libraries. You may think you know what you're doing. You don't - not enough to produce genuinely secure crypto.

Don't roll your own numeric libraries. There are libraries with decades of improvements to handle corner cases and increase stability. You won't create a better one, not in one lifetime.

So, yes, in some cases code reuse through libraries really does improve software quality.

It fundamentally comes down to this question: Can I, the user of this library, produce a library that is more robust for my purpose than the existing libraries? Those who say "yes" often underestimate the amount of care needed to create code that actually works - that works solidly, that is robust against failures on certain inputs.

But, you say, you don't actually have any of those inputs, so why do you care? You don't - now. But the day may come when your program is going to have to handle those inputs, and when it does, then it's your problem, because you wrote the library.