There’s a lot of stuff saved as user meta that has no business being sent over the REST API, therefore the default endpoints do not include every piece of arbitrary meta automatically. If you want a piece of meta to appear in the REST API responses you need to register it with register_meta(), with show_in_rest set to true:
register_meta( 'user', 'your_meta_key_here', [ 'show_in_rest' => true ] );