Open Whisper Systems (the developers of Signal) apparently don't want to replace it or even just build in a fallback [0], and even fought with the LibreSignal developers, which is a fork that removes this dependency, and told them to stop using their servers [1], which lead to LibreSignal being discontinued.
I don't know what the heck is going on with this, but yeah, it's not pretty.
[0]: https://github.com/WhisperSystems/Signal-Android/issues/1106
[1]: https://github.com/LibreSignal/LibreSignal/issues/37#issueco...
There's also a more elaborate article on the topic here: