Okay neckbeard, I'll spell it out for you.
The volley takes ~2.4 seconds to travel back to the goal line. Let's assume that the keeper is standing at the 6 yd line (5.5m), and needs to get back to the goal line to make the save. Keeper also has to move laterally since the ball isn't returned directly over her head. Hard to tell from the video, but let's assume half the width of the goal of lateral movement (8 yds / 2 = 3.7m). sqrt(5.52 + 3.72) = 6.6m of distance that needs to be covered in 2.4 seconds, or 2.75 m/s.
At this point you're probably scratching your dorito-crusted chin and thinking "aha! most athletes should be able to move 2.75 m/s." Let's look at the other factors in play.
1) Typical reaction time is about 0.2 seconds, so that immediately cuts the available time by 10%, down to 2.2 seconds. 2) Teammate is trying to flick the ball on (but misses), so the keeper is probably not expecting a strong return volley, likely extending the initial reaction time. 3) It's difficult to quickly judge the angle of the return in the first split second. If the volley is aimed lower, the ball bounces in front of the keeper and is a much easier save. If it's aimed higher, the ball goes out of bounds or lands in-bounds after a much longer hang time. Within the first second she doesn't know whether to be moving forward (to either intercept the shot on the bounce or challenge a very high but short trajectory that the other team can make a play on) or backward (to save the shot in the video, which only happens at a very specific volley angle). 4) It's not as simple as running 6.6m in a straight line to make the save. She has to travel back and to the left, while keeping her head turned forward to track the shot. 5) Even if the keeper beats the ball back to the goal line, there's very little margin for error on the save. Momentum is carrying both the ball and the keeper into the goal, so she has to accurately punch/deflect a ball to the side or over the top of the goal. That's with the ball traveling ~20 m/s (45 mph), coming from behind and over her shoulder at the time they meet.
We can't see the keeper's initial reaction in the video, but my guess is that she took a step either directly to her left or even a bit forward, to cut off a shot that she thought might bounce in front of her, or that might travel in high enough of an arc where she'd need to come out and challenge it against the opposing team. She quickly realizes the true trajectory and starts to move backwards, which is where we first see her in the video, trying to get back before a fucking BOOMER goes in over her head.
Lastly, dude. It's funny to say "bad keeper is bad" when you're watching a professional on TV who's getting paid to always make perfect decisions and execute them. Saying that about a 5 second clip of an amateur misjudging a perfectly aimed volley is shitty.
If you were on keeper's team and railed on them for missing the save, you'd rightly get called an asshole, which is why you're coming across as a socially maladjusted dweeb who's never played a team sport.