due to the new rules of European Union, whatsapp has/had one year - iirc - to open the app to talk with other apps. iirc, this started on February/March of 2024 and we are in 2025. Anyone knows more details or news about it? This is ONLY applicable to European Union countrys, iirc.
Signal owners’ position is that due to the way protocol works, hosting takes a lot of resources as compared to other IMs. Say, in group chats the same message encrypted differently is stored for every session of every user in that group. So if they made some integration, that would mean a lot of resources spent on people who don’t even use Signal itself, and the need to support that integration.