There’s DeltaChat which technically is an email client and uses bog standard SMTP/IMAP for the transport of messages. Great idea, as everyone is reachable via their email address. And even if they don’t have the app, they’ll get your chat message as a normal email they can reply to.
But for a while now they’ve decided to create their own server nodes (“chatmail relays”) and heavily promote the use of those instead of your mail provider’s. While those are also just SMTP/IMAP, you still need a different domain and account i.e. different username - which makes them just another chat network which happens to be interoperable with classic emails... for now.
My point is: There seems to be some tendency for things to go proprietary.
I have no idea about DeltaChat, but it's not surprising.
Social media and chat apps are successful because of the network effect.
Let's say I have ten friends. Nine of them use the same app exclusively and one uses another app.
If everyone want to stay in touch with all of them, the two most likely things are 1) the tenth friend migrates to the common app, 2) the tenth friend installs both apps. In both cases, most interactions use the common app, unless the tenth friend is so influential that everyone switches to the other app just for the sake of them.
When you want your app to be popular, you want to discourage people leaving your app for someone else and you want to encourage people to use yours (better features or, more commonly, a larger user base). As a result, unless there's any external force going in the opposite direction (regulations or just people really hating lock-in), you'll build your services so it's not that easy to leave. Better features are an alternative, but at some point new original features are hard to come by and can be expensive.
This is especially true if you want to make money through subscriptions, downloads or selling data, because the more users you have the more you earn, but it could also be true in free apps if the owner wants visibility.
Again, I don't know DeltaChat, but this is unfortunately the general trend.
I grew with the inverse assumption.