It's a daylight basement, however, exposed in the back of the house, so
I would expect if there was a rising water table the backyard would
also be flooded? Only it isn't. In fact, the back yard seems to drain
pretty well.

Not necessarily, surface water can dissipate pretty quickly if there is
no blockage or pooling on the grade. Water that soaks into the ground
from the surface has travel through, obviously, the ground: clay, soil,
rock, etc. Clay is especially bad because it takes a long time to
saturate and then a long time to dry out. Now consider the crushed
stone underneath your basement slab. Water can travel through that
pretty easily and most likely collects there if it cannot get out fast
enough through the surrounding soil conditions. It's not intentional
but the stone located below subterranean basement slabs acts like a
drywell in wet locations. Therefore a sump pump is required to keep
the water level below the slab.