Google could theoretically build a Google Messages counterpart to iMessage and skip the Carrier as the middleman, but then it wouldn’t be interoperable with iPhones since it wouldn’t be an “open standard”

Google did that, in 2013. Hangouts was briefly the default SMS client on Android, and it would upgrade conversations from SMS to its protocol when available. It was available for iPhone, but couldn’t be an SMS client there.

Rumor has it, carriers whined about it, and Google caved out of fear they would promote Windows Phone devices instead. I think that was a foolish move on Google’s part, but I think I’m glad Google doesn’t own a dominant messaging platform.