Because most people don't really understand the specifics of what they are railing against and/or arguing for. They just read "big bad Apple is being eViL" and get on the complaint train without really understanding what they are advocating for.
In my opinion Google should really being trying to push Chat as the default for Android instead. It is no more walled off than Googles RCS implementation, and unlike RCS they don't need Apple to bake it into their messaging app to attract users.
The fact that they care more about their RCS implementation for this purpose instead of Chat or just the basic RCS protocol as the standard tells me Google is more interested in the easy road to being the technical foundation of texting for reasons other than the universal convince of customers or the proliferation of standards.
In reality they probably want this so bad because if they were to get it it would make it effectively impossible for the average person to avoid Google services on some level, which is good for their data mining empire.