Honestly I think they simply don't care, and from their perspective they finally have gotten it right with Teams.
Software wise though idk if there is limitations baked into iOS and Android that limit this
Since I started my online journey I've been through ICQ, IRC, MSN, Skype, Hangouts, Kik, Teams, Telegram, Whatsapp, and many many others I don't even remember.
Yet at every point it seems that something breaks, they feel like they have to add more and more features, core features start sucking, etc.
Whatsapp is the only one that stuck since I started using it, and I suspect part of the reason is both the fact my friends and relatives just won't move from it (it essentially replaced SMS) and the fact that Whatsapp cares a lot about the client being very performing even on lowest end old devices.
Combined with not advertising it at all recently, and other services taking the spotlight ("Let's facetime!"), I'm not surprised that people have largely stopped using it.