Because mapArray's implementation expects a mutable array type

export const mapArray = <T, R>(mapFn: (item: T, index: number) => R) =>
  map((array: T[]) => array.map((item, index) => mapFn(item, index)));

using it with a readonly source array will make TypeScript complain with TS2345:

const source: readonly number[] = [1, 2, 3];

const source$ = of(source).pipe(
  mapArray((item) => item * 2)
  // ^ The type 'readonly number[]' is 'readonly' and cannot be assigned to the mutable type 'number[]'.ts(2345)
);

I would argue that any function consuming an array should most likely type the argument as readonly so both mutable and readonly arrays are accepted. Except, of course, if said function is supposed to actually modify the consumed array.