Because laser fire is like guns in a WW2 plane. When you fight with a large and unwieldy gunship (take for our world example an attack heli that doubles as transport) and let that fight against a small fighter jet armed with only machineguns. It's outmanouvred and outsped with ease, and we all know that placing additional turrets on planes slow craft only works to a certain degree (WW2 bomber formations)

That is why missiles are their only true defence or offence against fighters.