Because if the product is not in red (pretty sure Google Reader was in green), closing it burn user trust. I know about opportunity costs, but I'm not sure Google is in a position where it bleeds money so bad that need to rationalize things.
And having one more product helps you synergize strategies across the board.
99.9% of Google users are completely unaware of their reputation around closing down products. Closing Google reader a decade ago is a cause that nerds like us on HN care about, but virtually no one else. And the same nerds like us also care enough about our online presence to not use Google products for a hundred other reasons anyway
