GREAT question- it’s because of how spiders abuse fluid pressure for locomotion!
see, unlike other arthropods, spider legs are mostly hollow. and instead of the traditional leg muscle arrangement, why, they’re just full of blood!
what few muscles ARE there act as hydraulic pumps to shove fluid around in the legs, moving them through hydraulic pressure and hydraulic pressure alone. so when the spider dies, those muscles relax and the pressure drops, leaving the legs to fold into the natural resting position of their stretchy ligaments and joints, completely unpowered.
basically, spiders are just bags of fluid that move their various rigid parts around by raising the fluid pressure in small areas to extend and contract their joints as needed, like an assembly line robot.
to spiders, humans are an unnecessarily complex bag of wet levers that shouts a lot.
and you can actually see this in practice yourself- if you happen to have an intact dead spider handy, just pick it up and give its body a gentle squeeze! right now. squeeze your dead spider right now.
the pressure your big meaty monkey hands are exerting on the spider’s body moves the fluid out of the spider’s body and back into its legs, extending them in a horrifyingly undead fashion! exactly like those air powered “jumping spider” toys you had as a kid.
you’ll never be able to unsee this now. you’re welcome.
also, if that weren’t cursed enough, this complete reliance on hydraulic pressure for movement means that spiders are actually slightly pressurized at all times. have you noticed that when you squish a spider, either through accident or furious monkey malice, it kind of just goes “pop”? yeah, that’s why.
they’re basically just tiny water balloons with legs. overall, I’d definitely say that “bag of wet levers” is the better way to go.
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