Are they, though? Google absolutely could have implemented an iMessage competitor, directly in the stock Android Messages app, and then required third-party Android manufacturers to include it as the stock SMS/MMS/"gMessage" app as a part of Android conformance testing.
But no, instead they choose to play games with Allo, Duo, Hangouts, Chat, etc., all of which are an optional download and need not be included in the stock install. And even if/when they are required, it's still an extra app that a user has to find, and understand why they should use it.
Now, I don't want them to do this. I want them to promote and support a federated, open standard; I don't want another iMessage. RCS is not great for many reasons, but at least it's not a walled garden.
Yet again I recall the deal with the devil Apple did with AT&T, giving them a year or two of exclusive rights to sell the iPhone in return for having exactly zero control over the device. That was an excellent trade. Before 2007, carriers were intrusively involved with all aspects of a mobile phone.
The problem with RCS is that the solution has been stuck in GSM consortium hell for over a decade.
I mean...are they?
If Google were serious about pushing a new standard, and were willing to actually push it on the carriers, they have plenty of money, reach, and clout to make their point heard loud and clear. That would be triply true if it weren't a "new standard" that was yet another transparent attempt to gather more data from users.
And even if Google pushed a new, better standard (than RCS) on the carriers, Apple could (and probably would) still refuse to implement it.