Why is it challenging to manipulate points when working with zipped shapefiles in Go?

[ez-kraivit]

     shape, _ := shp.OpenZip(tmpFile.Name())
     defer shape.Close()
     for shape.Next() {
    _, p := shape.Shape()
    if point, ok := p.(*shp.Polygon); ok {
        fmt.Println(point.Points)
    }
}

Why do I have to write it as p(shp.Polygon)? I want to be able to write p.Points() directly. It seems like I have to spend time understanding the internal structure and designing according to what I need