I doubt if there is a single overarching reason. That said I can give you a few opinions/theories to mull over.

1. Peer culture - millennials are the generation that made texting cool.

2. Speaking to someone in a two way exchange in real time is too intimate, and moves too quickly. Leading to awkwardly long silences.

3. Having the written text is convenient, you don't have to remember what the other person said, you just read it again.

4. A ringing phone digs at you harder to answer, so a text gives you an illusion of freedom/rebellion.

5. A text message will go through even when the signal strength isn't good enough for a voice call to do so.

Pick whichever ones work for you, or add some more to the list.

Given how often actual voice calls are not people you want to talk to, e.g. sales calls, surveys, political calls, it isn't just the Millennials that have begun to dislike talking on the phone.

I sure don't know what it is, but I am on call (24/7) for the most of the time and young people seem to prefer to text, even in the middle of the night, even when it is an urgent situation. What gets me is how is the text chime supposed to wake me up in the middle of the night?