I'm not sure they aren't. All of my Google+ photos wound up in Google Photos without any work on my part, other than maybe actually accessing photos.google.com.
It's confusing to me, but trying to explain it to my mom is absolutely impossible. I wonder whether the average user even knows there's a difference between the two. It's a perfect example of unnecessary redundancy.
Google Photos was announced May 2015, explicitly as the future of what had been Google+ Photos. It has been clear since then that that's what would be supported going forward, and then Google+ Photos was deprecated legacy.
(Also, not to be picky, but I see no mention of Google+ Photos (let alone its status as "deprecated legacy") on the announcement blog post: http://googleblog.blogspot.com/2015/05/picture-this-fresh-ap...)
Look at the Inbox vs Gmail experiment for example. While not tied to Google+, Inbox runs parallel to Gmail and accesses the same things. Just a different window dressing. I'm sure many at Google want to see Inbox take over Gmail at some point.
This is one of the reasons I doubt we ever see a Material Design Gmail. I think they'll run with it until Inbox is "ready".
The latter is not a replacement for the former, which many many people depend on, so the only way Gmail is going away is if Inbox evolves to be it...
https://plus.google.com/+googleplus/posts/d1XubVAZ5hV
Don’t worry, all your photos and videos will still be saved and available after you switch to the new, stand-alone Google Photos app.