Google Play Games doesn't restore highscores?
Google Play Games Services support saving game progress since the beginning, and it's upgraded on November 2014. From Android Developer blogs,

[...], when Google Play Games launched, one of the very first features we included was the ability for users to save their game to the cloud [...]
[...] this past year (ed. 2014) at Google I/O, we launched an entirely new Saved Games feature, powered by Google Drive.

However, being able to save doesn't mean it will automatically save your game progress: the developer has to implement the logic that uses this service.
Anecdotally, I have played some (rhythm) games that use this service and worked properly when I replaced my Nexus 4 with Nexus 5. On your case, unfortunately, I have to say that this is fully the developers' fault.
As for why they use Google Play Games service, its features are not only for saving games: achievements, global highscores, etc.