It’s absolutely more common in Mastodon, and it’s a common feature of clients to prompt for alt text when a user is posting an image. It’s as much cultural, and is considered good etiquette. I agree that software would dramatically affect this, not by showing it by default (where, for people who don’t need it, it’s merely distracting) but by recommending it when posting. You’re absolutely right that it’d be more common if folks were reminded of it.