... which they have done, for some definition of "best".
They've chosen an implementation with good cache locality and lower memory overhead (and presumably better average-case speed) than the chaining implementation usually used for std::unordered_map. The downside is that its worst-case speed is pretty bad.
On the other hand, I believe they use Siphash by default, which means it's very difficult to even intentionally trigger worst-case behavior without being able to inspect the process's memory for the random Siphash key.