How does Apple being the standard chat app help them? They are a hardware company. They also make money through paid services, but since no one will subscribe to a messaging app when all the others are free, this isn’t viable either. The only way to monetize a cross-platform iMessage is through ads and/or data collection, which Apple doesn’t want to do. So how would they make money off of it? They wouldn’t. They might actually lose money because:
1. They would have to build out servers to support billions more devices, for free.
2. They would lose hardware sales to people who are locked in to Apple because of iMessage.
The mistake Apple’s competitors make over and over again is prioritizing market share over the part of the market that’s actually profitable. Apple is focused. It would (wisely IMO) rather sell expensive iPhones than be a loss leader with the dominant platform.
Also, can you imagine the antitrust investigations Apple would be facing if they owned the dominant chat app, yet bundled it with their OS? Right now Apple can answer the monopoly accusations by rightfully pointing out that they have minority market share.