"침대 아래에 거미 세 마리가 숨어요."
Translation:Three spiders hide under the bed.
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In short, because there isn't a subject marker attached to 침대, but there is a subject marker attached to 거미.
We know that 거미 is the subject because it is attached to subject marker ~가 through the animal counter 마리. And we know that 침대 is the location because it's followed by the preposition 아래, which is attached to the location marker ~에.
But also context clues tell us that it would be very confusing if the bed was a) actively hiding, b) underneath 3 spiders.
The sentence is flipped around to emphasize the spiders are under the BED, rather than under some other piece of furniture.