Lasers have a big power to mass+volume problem for mounting them to fighters. Check out the anti-mortar laser systems for an idea of the size of current laser systems required to shoot down mortars. That's probably a good lower bound on the power required for an active A2A missile defense system. Similarly with using kinetic defense systems mounting a system with a high enough rate of fire along with the tracking and pointing mechanisms just doesn't work on a small plane like a fighter, we have systems like that for ships but they're huge.