Why do the killers carry the survivors on one shoulder?

Okay...here we go.

Trapper has lots of experience with heavy lifting, as he did work in his father's mine. Heaving up a survivor to his shoulder should be no problem.

Wraith has likely been working hard labor jobs since coming to America, so he's definitely got some muscle. However, it's possible signs of back damage are already showing, as you can see whenever he is standing still he has his head tilted to the side, as that is probably to most comfortable position for his spine.

Hillbilly has worked manual labor his entire life, plus he's already got a hunch. I doubt his father or others taught him proper lifting techniques and he had to just go with what was most comfortable/efficient.

Nurse has some sort of telekinetic force that allows her to break pallets and generators, so I assume she must be using it to pick up survivors.

Myers is pure evil. He probably lift people up without proper lifting technique just to make people angry because he's just so evil.

Hag seems to already be crippled slightly due to the cannibalism. However, she probably has some sort of hex that allows her to be able to summon some ungodly amount of strength to lift survivors.

Doctor is filled with wires and other metal bits, likely reinforcing his body and allowing such fluid movement.

Huntress has years of experience transporting kills to her home and has built up quite a bit of muscle in the process. If she can drag a bear through the woods for several miles, she lift one person off the ground.

Bubba should know better, as he has had years of experience lifting people in order to have a nice family dinner.

Freddy is a dream demon and is likely using his nightmare powers to make us perceive improper lifting techniques. The true nightmare was spinal damage all along.

Pig has likely rigged up some sort of lifting contraption in her arm to help lift people up onto her shoulder. She was the apprentice to Jigsaw after all, years of mechanical engineering could allow for such a thing.

Clown is probably numb on painkillers and other drugs he can't even feel the pain his back is in whenever he lifts someone up.

Spirit is an incorporeal being and as such does not contain a skeletal system.

Legion are still young and rebellious. They don't care about proper lifting because you're not even their real Dad okay!

Plague is basically rotting from the inside out and her nerves can't even function properly. Bending down and lifting is the least of her worries at any given point.

Ghostface has always been a troll and is doing it on purpose just for the lols. He's so meticulous if he wasn't doing it for laughs he has some other reason for doing it surely.

Demogorgon... I'm not even going to pretend to know the anatomy of a creature from the upside down, so I'll just assume it's doing probably upside-downian lifting techniques.

Oni is probably just full of adrenaline all the time from all his rage he can't even feel pain. Even if he did, it would just make him angrier and more powerful.

Deathslinger is already crippled with that leg brace. He probably couldn't bend over if he wanted. That's the only way he can pick anything up.

Pyramid Head is technically just a manifestation of torment and improper lifting is probably just meant to torment all that witness it.

Blight is just filled with whatever is in his serums, which probably increases his strength and pain tolerance allowing for him to lift survivors effortlessly.

Charlotte had been carrying Victor around inside her chest for most of her life. Not to mention all that scrap and junk in her bag(?). With the deformity from being conjoined and the back strength built up from all that stuff, I doubt it's much of an issue for her to lift survivors.

Trickster just loves being the center of attention. Lifting improperly is just one of his many was for eyes to land on him. Don't fall for it, it's exactly what he wants.

Nemisis was designed for abuse. He can take gun fire, rockets, and even acid. The guy can probably one arm bench press a tank. He's probably reinforced with so much stuff nothing can stop him. Not even a pulled muscle.

Man that really stretched my creative thinking.