Why not? Jets are commonly shot down with missiles. It’s rather hard to kill them spraying them with machine-gun fire. You can do it of course but a missile is the more reliable solution which is why jets made for dog-fights usually carry multiples.

Because Iskander is not an air-to-air (which jets carry) or even ground-to-air (like the aforementioned patriot). It’s a ballistic missile. Ground to ground. Very different targeting systems and the rocket itself