"Default messaging app" is a creation of Android, necessitated because every cell phone manufacturer wanted its own messaging app. It somehow later became a feature people needed because those pre-installed apps were often dreadful adware junk. This was never a problem on iPhones. No one wants to set a "default messaging app". It mixes up where messages go. I want my Signal messages in the Signal app. I want my LINE messages in the LINE app. Putting them in random different places doesn't make sense and confuses where they're coming from. I don't want my contacts showing up half a dozen times repeatedly for every messaging app they're using.

I don't see anyone on Android wanting to put their SMS messages in the Discord app.