As I've become fond of saying... space is big. Really, really big.
By contrast, a ship is small... really, really small.
At best, you might see, just for a moment, where a ship is, and might be able to project a statistical "probability cone" from that measurement, but within milliseconds, the odds of a hit would become essentially zero.
So, the idea of a "blanketing fire" region is essentially useless in real-world, real-scale terms. Unless you fire at EXACTLY the instant that you see the torpedo-firing ship, and your fire arrives at essentially that same time (meaning phasers but not torpedos), you're never going to stand a chance of a hit.
You can't "blanket fire cover" a region of space consisting of millions of cubic kilometers. And that's what we're talking about for the speeds at which Trek ships generally maneuver.