It's about priorities. Signal's priority is security. If the current state of the art is you can chose security or foozles, then Signal doesn't have foozles. If people really
want foozles then they need to do the work to figure out how to deliver foozles securely and then Signal can integrate that work.
All of the other products have some other priorities, and so if security gets in the way, too bad security loses. Even WhatsApp, which uses the Signal protocol, has other priorities so the actual client software isn't focused on security, they were just happy to get a secure protocol out of the box.
The interesting thing is that years of research hasn't found many things that are just plain impossible securely, there are just a lot of unknowns or places where the secure option is harder. For example when you add a popcorn GIF to a chat Signal proxies this twice, masking your request from the GIF provider (Somebody used a popcorn GIF but the GIF provider doesn't learn who) and also from Signal's own servers (ishanjain28 used a GIF but Signal doesn't learn which one). Most outfits wouldn't bother, who cares about security anyway? But the feature now just works, without unnecessarily giving away information to people you might not trust.