It's almost certainly the carriers imposing this restriction. The reason these features took so long is because, as they did for SMS, every single carrier across the world has to get on board with this. , so I'm sure that's the reason it's not encrypted. The Messages app might be a Google product, but the underlying transport has to be implemented by carriers, and I'm sure there are many, many carriers who aren't interested in encrypting the messages that are carried across their networks (either because they don't have the technical competency, they want the data for themselves, or they have to comply with local law enforcement regulations about texting).

This is not to excuse the lack of encryption, but the finger should probably be pointed more at carriers (and maybe OEMs) than at Google.

One question I have is whether Google-account to Google-account messages will be encrypted, like how Apple does it with their iMessage/SMS boundary within the same app...