iMessage is an app. Android is an operating system. I think what you meant to say is iMessage doesn't support RCS.
The difference is Apple worked hard to keep it this way for decades, even so far as "patch" a fix that was created to make it possible for their customers to communicate securely with Android users.
And Apple is only going to support RCS because they were forced to, and they'll on comply to the degree that they think they can get away with. Just like they're doing with app stores.