I mean, technically, it was. Erso rigged it so that it blew up from inside the design team.
Also, I’m just gonna say it- R2D2 took the shot. He was faking being shot so he could focus. That little droid has made quite the career out of letting hicks take the credit.
Not saying it makes sense (but shooting at moving targets that can change direction after you shoot, from a gun mounted on a platform with movement out of your control is not just math)
I thought there was a line about calculating the route for hyperdrive and it taking a minute, so maybe they could but wouldn't do the math fast enough. Mean it sounds plausible for the 70s when Douglas Adams was going on about digital watches in his books, nowadays it takes a bit to believe that'd be the situation.
I mean, from an engineering and mathematical perspective the gun aiming problem and bipedal droid mechanics are very similar in a lot of ways, except the bipedal droid is much more difficult.
Ultimately, though, who cares? Nothing about Star Wars makes scientific sense and Luke shooting guns from a manual turret is more fun than a computer casually blowing up the tie fighters.