Though seemingly unintentional the placement of a CPU on a motherboard is a strategical design choice made by manufacturer's for various reasons. Most notably CPU cooling and internal device temperatures are taken into consideration when deciding the placement of a CPU on a motherboard.
The processor placement is not a requirement to be considered within the specifications of the ATX form factor or any other form factor for that matter. The processor is almost exclusively placed in the upper left to upper middle section of a motherboard based on the assumption that it will be in an upright tower chassis which will align the CPU with the rear exhaust fan and the power supply unit's cooling fan allowing for better cooling.
Due to the common thermal considerations taken into account by manufacturers when deciding CPU placement on a motherboard the location of any CPU across the majority of all consumer level motherboards has varied by a matter of one to two centimeters maximum in the last twenty years.
CPU placement on enterprise level servers which are housed horizontally rather than vertically like consumer computers can vary widely as there are fewer on board devices to design around and a different thermal pattern to work with.
Placing the CPU in the upper area of a motherboard reduces the probability of other devices or their data and power cables from blocking the airflow to and from a CPU. The CPU is also a large heat source which would cause thermal damage to any wires or cables crossing over it for extended lengths of time.
Given the recent affordability and ever increasing wide spread acceptance of liquid cooled processors it is possible in the near future that CPU placement will change drastically as liquid cooling systems remove far greater amounts of heat without spreading it around the inside of the computer's case as with air cooling. Thus water cooling removes many of the restrictions imposed by thermal management from system designers.