Slice and Splice - why use for loop?

JavaScript
1

Tell us what’s happening:
Its solution uses for loop and traverses through each item of first array but why? We can have a single line code instead:

localArray.splice(n, 0, ...arr1);

Your code so far

function frankenSplice(arr1, arr2, n) {
  // It's alive. It's alive!
  let localArray = arr2.slice();
  localArray.splice(n, 0, ...arr1);  
  console.log("localArray = "+localArray); // shows: localArray = 4,1,2,3,5,6
  return localArray;
}

frankenSplice([1, 2, 3], [4, 5, 6], 1);


**Your browser information:**

User Agent is: <code>Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:63.0) Gecko/20100101 Firefox/63.0</code>.

**Link to the challenge:**
https://learn.freecodecamp.org/javascript-algorithms-and-data-structures/basic-algorithm-scripting/slice-and-splice