Locked out... again

I seem to do this frequently, although this time it happened a little differently (and not nearly as dramatic). So earlier in the day Jack was being naughty so I put him in timeout in his room for a few seconds. The worst possible torture you could ever inflict on Jack would be to put him in his room and shut the door, he could have food, drink, tv, and toys in his room with him, but once you shut the door the world has ended. Anyways he is in time out and I can hear him trying to reach the door knob but I know he cant ge to it. So I take him out of timeout and we go on with our day. That night I put him in bed and shut his door. I go about my evening until I decide its time for bed. Dan goes in to check on Jack..... the door is locked. Yes, this poor child (although asleep) is locked in his room. How does this keep happening to me?? Thank heavens all we needed was a screw driver to get the door unlocked. My question is... Why would someone build a house and put door knobs with locks on the spare bedroom?