I could think of a few reasons out of the massive number of reasons why. For starters, its another service that ties user to the brand. While it probably won't be a huge money maker for them, there are tiny things that may be worth doing to help them leverage growth and not to mention more analytics.
They wouldn't gain any new analytics. As a registrar, Google already has access to the root zone files, they can see what domains are being registered/dropped at every registrar. They already run public DNS servers too, so they don't need to get people to register domains with them and use them for DNS to see DNS activity.
I agree but arguably, you could say one of the focus is the cloud. Building an AWS competitor isn't necessary something they wouldn't consider in the future. They do have a DNS service, and they already do 3rd party registrations with Google App integrations for various things. None of those have been on the cut list. And I'm sure they have plans to continue pushing things forward for cloud services. Being a registrar and having services integrated with each other (see AWS) is an added benefit. I could be wrong and this is purely random thoughts out loud, but not everything should be viewed so narrowly or direct that you miss the big picture on where the focus is at and how you get there. It's still a possibility and none of us knows what Google has planned.