There is a hint in the name i guess. after all, they are ground spoilers, not flight spoilers...
Usually those are design decisions between the need to dump lift on landing to achieve the required deceleration and the need for usage in flight where probably less of a spoiler control input is needed to achieve the design goals of roll augmentation and deceleration/drag increase if used as a speed brake. Of course those design decisions have other implications as the wing structure has to be designed to withstand the added stress of spoiler deployment in flight.
By the way, not a unique design of the 737, other aircraft have similar designs, for example on the A320 the spoiler 1 pair is used as ground spoiler only, as usual though, it is a bit more complicated there, as the spoiler pair 5 cannot be used as speed brake in flight, but roll control only, however it does act as a ground spoilers, as do the ailerons (aileron anti-droop).