The 'why' part is easy to answer: Apple uses SSD (Solid State Drives) in their computers because they are many times faster than a hard disk drive, they are smaller and lighter, they use less power, and they do not have the same reliability issues associated with having a spinning drive in a computer than may get knocked or dropped.
One of the reasons why they want the drive to be physically smaller is because the rest of the laptop is pretty much full of battery.
Apple still sell one laptop that has an internal hard drive: the non-retina 13" MacBook Pro. The base model of that machine has a spinning hard disk drive, although it can be upgraded to an SSD (and it would be strongly recommended, for performance reasons).
You can not fit a hard drive internally to any other MacBook, MacBook Air or MacBook Pro, because there is not the space or a connector for it. If you need to use a hard drive with these laptops, you would need it to be an external drive, connected over USB, Thunderbolt or across a LAN.