Because if you can just communicate around a walled garden, what’s the point or value in staying in one?

Because people are happy with that garden and don't think about others. Please remember that your average internet user doesn't really know what an API is, or understand about open standards, they just want to find some content that matches their interests, upvote and share said content with their friends who are also inside that garden.

This average user isn't a bad person, stupid or naiive, they just have other things going on in their lives and the internet is a small part of it. They use it, take what they want from it and move on, and there are so many more of those people than you.

People who switch from iOS to Android report losing friends who were on iMessage and are unwilling to move to something platform agnostic such as Signal or WhatsApp. I wouldn't underestimate the walled garden effect.