It's illegal tying[0]. Google used to force Chrome and Google Search as part of Google Play Store requirements. And were fined a few years ago by the EU[1]. Pretty much most of this reasoning could be applied to a messaging app too.
Why didn't Apple & iMessage run afoul of this same thing? Is it because Google was forcing third-party manufacturers to include things, but Apple of course just doesn't have that issue?
[0] https://www.ftc.gov/advice-guidance/competition-guidance/gui...
[1] https://ec.europa.eu/commission/presscorner/detail/en/IP_18_...