you'll notice when "jiggling" your GPU while it is installed that it still has some leeway for movement.
even while screwed into it's rear extension card slot and it's PCIe slot lock in place the card itself can still move somewhat in it's PCIe slot.
i've seen a couple instances of people actually breaking their PCIe slot or the GPU's connector by knocking them or even just moving their system around without being extra careful.
Yep, they even slowly wiggle themselves loose just from temperature cycles and their own weight.
Also had a slot processor adapter back in the day with a similar problem. CPU card would up and fall out of the slot every few months because of the weight of the heatsink. Glad they discontinued that idea.