With only third party options for voice chat, while it can be demanded for ultimate, it usually isn't demanded for extreme trials or savage pugs. If there were an in-game voice chat, people would demand it in almost every party finder content.

I personally already have a hard time talking to other people irl. But talking to strangers in a game would be even worse for me. Not everyone can do something like that. So I like that there is a great option with discord for those who want to use voice chat, but there is no forced in-game option. So I'm happy that there is none.