Why are doorknobs in the center?

Placing centre doorknobs means that you have a thread on the rear side of the same and the rear side has a bolt. These are larger than the normal pairs of the same and there are no space issues and may even not need any latch fit through it.
Centre knobs happen to be larger than the standard pairs of knobs as these have no space issues that may otherwise not fit through any latch. However, where you have a doorknob screwed to the door and it acts as a pull that must be secure as one that is able to bolt from the back and these could work loose over a period of time.