Because you trade privacy for convenience. You could have a totally private communication platform, but you'd need to trade current IP addresses of your devices if there's no users and no centralized routing server or at least a list of what device is associated what person.

It's secure because people can't read the content of your message. It's not private because people can find you with your phone number or username and associate encrypted message packages with the sender and receiver so they know who you called and when, but not what you said.

So if your contacts are tech savvy enough to call you to get your current unique IPv6 address, something that Android doesn't really support out of the box, and IPv4 often won't work due to layers of routing caused by the world running out of addresses, or some other unique network identifier, and there are no firewalls between you or they've all been configured appropriately to allow the particular message protocol then you could send simple IP Messages to each other.

But as long as you want to use a system that routes messages and has a user database, that central location will always be a privacy hole.