And absolutely zero users care about the reasons. They only know that sending messages back and forth is dogshit.
The source of the lack of support across is Apple not wanting to even try because they want everyone to use their proprietary system on their devices instead. Google at least implemented a system to get RCS support to as many devices as they could, even when carriers didn’t do anything to help. Apple instead had to be threatened by regulators before they even began to consider looking at it.