为什么数组的协方差不适用于原始类型：Object [] arr = new int [] {1,2}？

Question

为什么数组的协方差不适用于原始类型：（对象<=原始）或（widened_primitive <=原始）？

Object[] arr1 = new String[] {"hi"};
Object[] arr2 = new Integer[] {1,2,3};
Object[] arr2_1 = {1,2,3};  // how is different from arr5 below???
Object[] arr3 = new Double[5];

Object[] arr4 = new double[] {1.2, 1.3};  //ERR:  cannot convert from double[] to Object[]
Object[] arr5 = new int[] {1,2,3};       // ERR: cannot convert from int[] to Object[]

double[] arr6 = new int[] {1,2,3};       // ERR: cannot convert from int[] to double[]