Because your example is a breaking change, and breaking changes are hard to make in a runtime that needs to reasonably support two decades worth of web content.
For example, if you have a `binarySearch` function that returns -1 if an element isn't found, a developer might do something. `const result = arr[index]; if (result !== undefined) { ... }`. This would then start returning the last element instead of undefined at that index.
For example, if you have a `binarySearch` function that returns -1 if an element isn't found, a developer might do something. `const result = arr[index]; if (result !== undefined) { ... }`. This would then start returning the last element instead of undefined at that index.