Because protecting their lucrative ads business means preventing a competing hardware/OS company from gaining majority market-share and strangling you with its platform. That's why the very expensive (not profitable) Android project exists in the first place. And when such a competitor gains too much power, your ads business can suffer very real non-theoretical harms [1].

Right now Apple iOS marketshare is increasing: it's reached 60% in the US and an amazing 87% among teenagers! (Who tend to "stick" with a platform even after they become adults.) And while there are many features that are driving the push towards iOS (it's not just messaging!) the dominance of iMessage is definitely one feature people [particularly younger ones] care very much about. And while fixing messaging might not be the only problem here, it's relatively "inexpensive" to run messaging correctly compared to the cost of fixing things after your OS market share crosses a tipping point.

[1] https://www.forbes.com/sites/dereksaul/2022/09/06/apple-cras...