To put it simply, because different parts are used for different things! In general, a large yard will tend to have three areas - a Receiving Yard, a Sorting Yard (likely a Hump Bowl for the largest yards), and a Departures Yard. In addition, there's likely to be a loco servicing area and somewhere to repair cars - those can sometimes be significant facilities, and your image shows some large repair shops in the middle (the long, large buildings). For a very large facility, you may even have two sets of Arrivals, Sorting, and Departures yards, one for each direction, whilst smaller locations can combine some or all of these into a single yard.

Trains arrive in the receiving yard, where the cars are inspected and the train is made ready for sorting. The incoming locomotives will go off for servicing, and eventually shunting locomotives will take the train and sort it. Once it is time for a train to depart, the relevant cars are collected from the sorting yard and taken to the departures yard, where the train is made ready for departure, the brakes checked, and the locomotives attached.

You might also have other areas, such as intermodal, automotive, or bulk cargo loading facilities. Alternatively, some yards have a very scattered design that doesn't fit this sort of model very well, due to age, subsequent expansion, and/or limited space.