With the phrase "God works in mysterious and unknowable ways" you have your answer.
Humans, as bipeds with insufficient skeletal systems that closely mimic qudrapedal skeletal systems (even have the "tail" bone on our rear-ends), are meant to suffer the resultant pain and agony of being supported by said skeleton.
Why? You cannot know and shouldn't be asking. Suffice it to say that God wanted all of us to be unhappy in this regard.
Same goes for any other number of seemingly incomplete designs. i.e. The Flightless Penguin with useless wings (on its way to becoming completely aquatic?)...The Ostrich/Emu with similarly useless wings...Whales with hind leg bones completely enclosed inside its body...Nipples On all male mammals (what is that about anyway?)...the list goes on and on. The only answer from the ID camp....God wanted it that way.
EBUCK