This is a perfect example of a comment that undervalues simplicity and ease of use (https://news.ycombinator.com/item?id=9224 is another one).
They work so well because the barrier is extremely low: it's literally a single interaction, keep the "button" pressed and the message is sent automatically upon releasing it.
And yet nobody was using voice messages when you had to: - Tap "attach" - Tap "audio" - Tap "record" - Tap "confirm" Or, worse, use a separate app and "share" to WhatsApp.
UX is important. I assume this service is based on a similar principle.