UICollectionViewControllerのCellにUIImageViewが表示されない。

class ViewController: UICollectionViewController, UICollectionViewDelegateFlowLayout {

let imageView = UIImageView()
let img = UIImage(named: "image")

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.


    imageView.image = img
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

override func numberOfSections(in collectionView: UICollectionView) -> Int {
    return 1
}

override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return 3
}

override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath)

    cell.contentView.addSubview(imageView)

    return cell
}

}

上のようにしても、コレクションビューに画像が表示されないのはなぜでしょうか。