I don't think you've seen enough people using WhatsApp to send voice messages to their contacts. I do see a lot of people doing that on the subway, the bus, the street, and everywhere else around Barcelona. And it is the same in every other place I've been to recently.

And I can see a very obvious reason why: it's easier. Typing is annoying and slow, and you make lots of mistakes. People don't care others are listening. When was the last time you memorized something irrelevant someone else next to you was saying on the phone?

Maybe you don't want a single word of what you talk about to be heard by anyone else. But most other people don't care, and the reality is that most other people around are too bothered with their own lives to pay any attention to it.