Because they fail test screenings. The general movie audience does not respond well to realistic depictions of explosions or cannons at a distance.
From the other SE, a partial excerpt of a great answer (that you should upvote as well):
Most film is art, not life/reality. Sound designers have to match the visual art on screen with the sonic art of their mix, and that usually includes a certain amount of "realer than real", suspending some of what we "know" about physics in order to tell the story in a way that translates to this 2-dimensional, 2-sense media.
Case in point, when we see some big impact on screen, such as a lightning bolt, we as audience members expect to hear it, much as we expect to hear the starship Enterprise whooshing by at warp speed, even though there's no air (and thus no air movement to create the "whoosh") in space. We also expect to see it match up with the visual effect on the screen; this both increases the overall impact of whatever just happened, and keeps us suspending disbelief that we're not watching a movie, we're looking through a window into another universe. Hearing a delayed sound effect might jerk us out of the director's world and wonder what's going on with the media system, and once we've left, it's hard to get us back.
The speed of sound delay would also, even in the ideal, make us feel further from the action. You see a lightning bolt hit a distant mountainside. 3 seconds later, you hear an ear-splitting thunderclap. While that might be totally accurate to the scope of the shot, in the ultimate ideal of the audience totally suspending reality and believing they're physically in this world you've created, they'll be thinking "damn, I'm glad I'm not over there", when in cinema the location of the camera can jump from place to place as needed, and the very next jump cut could put the audience right in the thick of the aftermath of that thunderbolt.
You'll also see sound designers architecting a "double explosion". They'll engineer a big, powerful explosion sound right at the moment you see it, then the visual effects guys will put in a massive shock wave, and as that passes by the camera the sound designers will add an extra "whoosh". Or, with a slight delay (and a really fast shockwave), the designers will engineer a small crescendo to the big boom, so you know it's coming. These little audio tricks aren't what you hear in comparable "real life" events, but then again, neither is the sound of a movie punch or a movie explosion compared to the real thing.
Slightly related, the Mythbusters episode 165 "Blow your own Sail" had a clip titled "Sounds Bogus", where they compared actual explosions to movie ones:
In a recent episode of "MythBusters" on Discovery Channel, show hosts Adam Savage and Jamie Hyneman worked with Meyer Sound to investigate whether the "big bangs"—specifically gunshots and explosions—heard in movie soundtracks are sonically faithful to the real events. The episode, entitled "Blow Your Own Sail," was aired on May 11, 2011.
For the recent episode, [the hosts] reunited with "Honorary MythBuster" and Meyer Sound Staff Scientist Roger Schwenke, with the goal to compare highly accurate, on-the-scene audio recordings to their Hollywood counterparts.
"Real gunshots and explosions are too loud to reproduce exactly in a theater and would damage people's hearing," says Schwenke. "So the question is: are movie sound effects simply the real sound played back at a safe level, or have they been changed in ways other than just level."
The results showed that the sounds are faked in movies. So aside from making the sounds seem "real", there's a safety concern too.