Mimics?

[UMS Author Lava]

I think...
The reason that the basic enemies in the Mario platformers disappear so quickly...
They're actually Mimics!!
Mimics are the "mimics" of the living. Kinda like Heartless.
We'll start off the chronology with platformers...
SUPER MARIO BROS.
The enemies you see are Bowser's special Mimics that imitate the friendly Koopas and Goombas and other guys from other towns. The "Bowsers" in the first 7 worlds are Mimics.
SUPER MARIO 2
It's safe to assume that Wart's machine makes Mimics. The mini-bosses, however, are NOT Mimics.
SUPER MARIO 3
OK, every enemy here is a Mimic, except the Koopalings and big Bowser-daddy himself. Yes, that means Boom-Boom is a Mimic.
SUPER MARIO WORLD
Ain't it obvious? But to clear up the Koopalings disappearing...
Iggy, Lemmy, Wendy, and Larry ran away really fast on the surface of the magma, which the programmers wouldn't show. Morton Jr., Ludwig, and Roy all warped to the doctor's office to fix the bruises on their heads.
SUPER MARIO WORLD 2
YAY. All the enemies here, except Kamek and Baby Bowser, are in fact Mimics. BTW, the Mimics that people call "Mousers" are really named Squeeks. See Paper Mario 2 and have Goombella Tattle at the Squeek at the Pianta Parlor.
SUPER MARIO 64
The mini-bosses, except Bowser, are also Mimics, though they speak. The REAL Big Bob-omb is at Shroom City, seeing to some affairs.
LUIGI'S MANSION
The ghosts that King Boo sends after poor lil' Luigi (*I press the A Button* MARIOOOOOOO!) are Mimics. What else am I saying to you?
SUPER MARIO SUNSHINE
Obviously, some of these guys are Mimics spawned from sludge. Bowser Jr. even tossed in a couple Petey Piranha Mimics. The one you see in Bianca Village's Petey Piranha Returns is a backup Petey Mimic.

And now the RPG's...
SUPER MARIO RPG
Almost all the enemies are Mimics. Except the original Mack, Bowyer, Yaridovich, and Axem Rangers. They just quit their jobs after you whipped them. And as for the Snifits and Apprentices, they aren't Mimics. However, the Spookums are. And when you face Magikoopa, he creates Mimics, as does Belome of you. And the treasure chest Mimics have the power to spawn Mimics. And the factory Clerk, Manager, and Director are just machinery. Dodo and Johnny, 'course, aren't Mimics, that's for sure...You get it! Almost anything that attacks is a Mimic!
PAPER MARIO
The only enemies that aren't Mimics are the Chapter Bosses(but Lava Piranha is), Kent C. Koopa, Jr. Troopa, Bowser, Kammy, and one Anti Guy(he just goes to the candy store after you beat him). In the Toy Box, Kammy makes Mimics.
MARIO & LUIGI
Trunkle, Mom Piranha, Piranh Bean, and all the basic enemies are Mimics. Chuckola Reserves was transformed into a Mimic(Chuckolator) until it was beaten down. Jojora's friends are NOT Mimics.
PAPER MARIO 2
Almost all the basic enemies are Mimics, except the fighters at the Glitz Pitz, the X-Nauts who talk, and Gus. The only Chapter Boss that is a Mimic is the Smorg cluster.

...And I am FIRM on this theory! But comment along if you wish.

[boomiester]

I always assumed that enemies disappeared to keep the screen from becoming cluttered with "dead" enemies, and because some of the earlier hardware (NES,SNES) would lag if sprites stayed on screen all the time.

[Lazo]

It's for simplicities sake. There was never any reason to keep the bodies of your enemies on screen, and no reason for elaborate deaths. Because there are multiple version of the same creature doesn't make them your "mimics." Similarly, there isn't only one human and everone else is a "mimic" of that one guy. There isn't much variation between basic enemies because those are unnecessary details, and the extra space was used elsewhere.

[Graedius]

Yeah, how about a logical explanation -- the console in question can only show so many sprites at once. To prevent intense slowdown they remove the sprites.

[UMS Author Lava]

It was just a weird theory of how that'd make sense in the platformer's world.

[Lord of the Dance]

Lol. In "Paper Mario 2", most of those basic enemies were wild animals that you ran into. I don't think those would be "Mimics"...

[UMS Author Lava]

Actually, they'd be 'ruffians', taking heed to what Prof. Frankly said, but maybe the Piders and Arantulas are real. But don't ask why a bunch of ruffian Koopas are in Hooktail Castle, 'cuz they don't wanna be sacrifices. I'm mainly talking action games.