Strange thing is this was always the case and just recently started to change. MSN API was a joke, you needed a desktop environment on the server to run the actual client to use SOME of the functionality. Nearly the same with Skype. Gchat removed Jabber pretty fast. Facebook chat also removed the outside functionality of their XMPP clone. Whatsapp is not even able to provide a web client, but only a weired bridge because they crippled to xmpp they are using way to much. No experience with the Apple services but i assume they are similar.

Next to that we always had open protocols, that worked perfectly, had plenty of clients and apps and nobody used them.

It is a stupid market.

Convince them to get Telegram. It works on most devices, and has extensibility built in:

https://telegram.org/blog/bot-revolution

https://core.telegram.org/api

I just installed the Telegram app. Why is it asking for my phone number?

They use phone number as account name rather than email on mobile.

What if I don't want to use it on a phone?

The web and desktop versions can send an SMS to authenticate if you don't want to or can't use the phone app.

https://web.telegram.org/#/login

I see. I don't really want to tie it to a phone number, though.

Already did with the in-laws. :-)