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Motion blur and the way the human brain processes movement, keyframing in some situations (the brain filling in the blanks of what literally isn't there) means things that are taken for granted in even a fairly amateurish still image are very much overkill in motion, wasting processor time and creation time.

I'm not sure I follow you. Please clarify.

Okay.  Imagine a 4 frame animation.  Frame 1 is a person with the arm stuck straight out, Frame 2 is the person with the sticking at a 45 degree angle up, Frame 3 has the arm straight up, Frame 4 is a copy of Frame 2.

Play those those 4 frames in a loop and the human brain sees someone waving their arm, filling in the motion required to get between those images.  You're not actually seeing the arm move at all.

Now assume that the motion being shown takes up a single second.  Use 30 frames to cover the movement.  In order to show a true representation of that movement, more detail and accuracy, you need more frames.  Say you shoot at 300 frames per second.  By definition, you've got 10 times the detail.  You have by far a more accurate depiction of the movement because you can see more detail on the exact arm position at a given time, see things like how the motion through the air affects the way the sleeve moves, the way the shadows being cast move.  Go to 3000 frames per second and it's more accurate still.

If you just want to show someone waving their arm at normal speed, it's also a complete waste of time.  All that detail is wasted because 30 frames per second is entirely adequate to show an arm moving in what looks like a lifelike and smooth way.  Adding 10 times or 100 times the detail doesn't help in the least because people simply won't see it.  Your shadows don't have to be accurate because they move too fast for people to make out details.

Instead of a person waving, imagine a person running.  The shadow on the ground while they are moving could be an indistinct blob without much definition and it won't bother people because they don't expect to make out the details anyway.  Going to all the trouble to make the shadow accurate, or reflections or highlights, is a waste of time for the same reason that shooting at 300 or 3000 FPS (and not for special effects) is a waste of time.  As long as things are consistent (lighting is coming from the same way, that sort of thing), humans are much more forgiving of lower accuracy.

On the other hand, if you want to do a render of a single instant of that movement, freezing it in place, then you do need that detail.  You can't cheat nearly as much because for a still image people will see that the shadow is wrong and won't accept an indistinct blob if there should clearly be a defined shadow.

To put it in terms of special effects, although having a film of a giant monster eating New York City is harder than rendering a single image of a giant monster eating New York City, it's easier to make the film look more realistic than a single image.  With the image you have to get the shadows all accurate, the lighting all accurate, the distance and colours accurate otherwise people will be able to pick out the inconsistencies that make it clear it's a fake.  In the film, people don't have the time to pick out those problems so the SFX artists don't have to waste their time on making it perfect.