Why does Milton feel it is not necessary to put up a monument in stone for Shakespeare?

Milton, in his poem, feels that it is not necessary to put up a monument in stone for Shakespeare because he had become immortalized in the hearts of his readers. He has built a monument for himself through his own works which flow easily and his readers receive his words as one would receive the words of God. Therefore, his words create an everlasting memory in the minds of his readers.