Same reason people use any social network: because your friends, family and/or people you otherwise/somehow care about are there.

If they're on Instagram, aren't they necessarily also on SMS, plus a billion other people? All ad-free?

> If they're on Instagram, aren't they necessarily also on SMS

Many people have friends in different countries, and while intra-country SMS is usually free, cross-country ones usually aren't. I think out of my current friend circle, literally all of them have at least one friend outside of the country where they live, but as an immigrant I'm probably biased too.

SMS is stuck in the past, even with RCS. Apple has used SMS as the wall of their walled garden for years. As an android user, it was extremely annoying to text to Apple devices (and still is to some degree)

If they’re on instagram they’re already in the Facebook ecosystem which means WhatsApp should not be objectionable and it covers the messaging aspect quite well.