Slice and Splice - why use for loop?

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);

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**Link to the challenge:**