Because Google will remove it from their index when they revisit the page organically. If Google provides a way to programmatically send removal requests, I'd be happy to hear about it and I will pass it along to the team that deals with account deletions so we can make it part of our flow.

Right now, I am only aware of the Google webmaster tools where you have to input each individual link you want removed, and even that request is queued and takes time, with no guarantee that they will reindex the content.